Stocking For Protecting And Heading West
The longevity granted by each Transcendent Immortal Tribulation was varied, but it was almost never lower than three thousand years and was often more than six thousand years. As for the upper limit of this, it could reach up to fifteen thousand years!
As for what caused this difference in longevity? There were many theories behind it.
The most accepted theory though, was that the more stable someone's foundation was, the better longevity they would gain from a Transcendent Immortal Tribulation. But there were other things that added to this too, such as the type of the Transcendent Immortal Tribulation one would have.
If it was a variant that was stronger than the normal ones, it would usually grant more longevity. But it also reduced the rate of survival, thus making it a less attractive choice.