Not very long by fantasy standards because the characters are long living youthful magical beings.
I was correcting the tone, 20, 30 years is different from 5,10, 24 years suggested by the elder. The prior seems cynical while the latter is more optimistic. The smiling and laughter from the elder, Qing Yun, and Wu Xin in the scene represents a bright outlook that the two males will return for certain. It’s not that they can’t open the seal, but that would risk the demon energy from breaking out. The elder was saying that the demonic energy will stabilize in 5,10,24 years and the two would be free to come back without dooming the world. Since Ding Yin and her father is alive and will be back Wu Xin is no longer suicidal. Followed by a relationship closure scene from Qing Yun. It’s bittersweet rather than cliche happy ending that leaves no room for a sequel. This ending had a high note given that the average Chinese fantasy dramas ends tragically for the main cast, looking at Chinese paladin.

I felt it was a decent drama given that modern dramas of this generation is by average subpar compared to the 90s. BTW Ding Yin is such player. My only complaint is that Ding Yin is not funny compare to Xiao Zhang and Wu Gui. I wouldn’t mind watching a spin off with them. There is a clear gender role reverse the female lead is the persistent one while the male lead is the tsundere. I felt it was refreshing that ZLY played such a strong female because she usually plays the soft and passive characters. If anything I am keeping this on my hardrive for her scenes.

I think you are interpreting the ending in a cynical manner, contradicting the optimistic tone given off by the characters and plot wist. The characters weren’t being positive they were certain of the reunion and acted according to the future. The elder suggested that their release would be within 5 to 24 years and ensured that Wu Xin will see them again. Qing Yun fully expects Ding Yin to be back. Hence, she resolved the triangle relationship so that Ding Yin wouldn’t be burden in the future. Wu Xin plans to cooperate with her sister in Xi Yu plains to bring a better world for Ding Yin to come back to. All characters are prepare for their inevitable reunion. She didn’t lose them. Accurately speaking, she is separated from them for a time frame.

You need to keep in mind that Legend of Zu is a Xianxia. Xianxia novel characters have special powers and extended lifespans. Some live 300 to 500 years and retain a youthful appearance. Xianxia literally means immortal hero, an exaggerated title given to characters have have powers beyond humans but below gods. The more powerful the character the longer their life span, and greater their powers. Hell even Wuxia characters lived in high 70s to 90s with youthful appearance and inhuman strength, Wuxia is considered less fantasy than Xianxia.

An example, Lu Pao waited 20 somewhat years to enact his plans and met his wife again. He retained his youthful appearance with exception of facial hair. Actually all the characters look very much the same despite the 20 year gap. The only thing that change for the men was facial hair and the women looked identical. Xianxia people are youthful long living people.

The elder literally guaranteed their return to Yue Er. They are alive/well sustained by the energies and the demon stones will always balance, The seal will open up when it balances.. This was the “good news” that stopped her from being suicidal. They are guaranteed to be free between 5 to 24 years. This news was what motivated Qing Yun’s and Yue Er actions. The three characters were smiling from ear to ear during that scene. The viewers personal experience had nothing to do with that plot point that was clearly directed to be optimistic rather than pessimistic. Unless you bring personal experiences to the drama.

You do know that legend of Zu classified as a Xianxia novel/drama? This is why they have magical seals, flight(almost everyone), sword spirits (anyone with a god sword), magical martial arts (everyone on mount zu and other mounts), see through walls (Wu Gui), turn into bats (Lu Pao), Animal morphing, Gaint Animals, mind control people with seals, disappear when they die etc. And you are doubting extended life which is consider the most basic of cultivation skills? Those feats are common in Xianxia genre. It’s like reading dc comics and complaining about superman’s flight, green lanterns ring etc. These fantasy elements are literally the foundation of Xianxia genre.

Not all are equal in terms of cultivation skill, a reason why Lu Pao was such a threat. The elders are older because their internal energy weaker than Lu Pao or they reached their height at a much older age. Youth various with time of individual cultivation height. Another troupe is once the cultivation reached new heights they stop aging or aged slower. It is obvious that the protagonist and other main cast have greater cultivation than your average cultivationist due to plot.

The higher the cultivation skill the greater the strength, longer the lifespan, and youth. This is a common troupe in both wuxia (inhuman) and Xianxia (magical). This is why Zu Er’s mother and brother looks more like her brother and sister.

Yue Er and Qing Yun are wielder of godly swords that would require high cultivation utilize. Yue Er was frequently mentioned as a high level cultivationist among the demon sect. Most of the characters that belong in any sect or mount are not ordinary people. In the ending, Qing Yun becomes mount zu leader. She would have to either already have the cultivation to hold that title or eventually gain the cultivation required. Longevity is literally the first step in cultivation before using all that fantasy stuff.

The only times they were close to death because of injuries, poison, external factos, etc. This had nothing to do with longevity from cultivation that is common in Wuxia and Xianxia. They are long living, but are not immortal. They are xianxia in human terms aka beyond human heroes to be exact.

1Failing to protect their students and their own sect is independent from their knowledge of the Demon Stone. Those are two different things. The stone’s powers are well known to elders of mount Zu since they have been guarding and studying it for generations on mount Zu. They are the living experts on it. The elder’s fighting potential has nothing to do with their knowledge about an object.

2)Elder, Yu Er, and Qing Yun were smiling with joy because they discovered that the two were not dead and will be released based on predicted stone’s behavior. The seal will open up once the stone has stabilize. It’s guaranteed they will be released. The only variable is when, but we know it’s within 5 to 24 years. This is well within average cultivist life time. Much less a highly cultivated Yu Er who is wielder of the Godly Blade that once sealed the Demon clan Xi Yu from Zhongyun. Qing Yun also wielded equally powerful godly blade of her own.

3)Cultivation level has nothing to do with length of training time. The height of cultivation is based on skill. Some grasp the basic much faster than others and reach their height sooner. Obviously the author favors main characters because they are normally “geniuses”. The sword spirit child is older than the great sage that founded mount Zu due to quicker Cultivation as a spirit. BTW, She is now tamed by Qing Yun. Lu Pao, Tu Mei looks younger than the Wu Tang clan head that belonged in the same generation (one that took the spot after Lu Bao killed the former head of Wu Tang) This is because Lu Bao and Tu Mei had reached their prime years before the Wu Tang head had reached his.

4)External Factors don’t affect longevity when they are cured or treated by “magical” methods. Cultivation extends life span. This is a norm for the Xianxia genre. How many times have these characters been fatally wounded only to be fine later on.

The master of Lu Bao is an example of well beyond his mortal lifetime. When he died their was no body because his life was sustained by magical cultivation rather than a physical body. This is why sometimes enemies die and turn to smoke because they aren’t surviving on a physical body alone.