Although a Ninth Stage Tribulation Transcender and an Eighth Stage Profound Sage were only one realm apart, the difference in lifespan between the two was huge.
The lifespan of an Eighth Stage Profound Sage could reach up to a million years. But most Profound Sages lived to around 500,000 years, while short-lived ones generally lived for one or two hundred thousand years.
This mostly depended on the Profound Sages' understanding of the 'laws' and their type.
It was also worth mentioning that all Eighth Stage Profound Sages knew secret techniques to burn their lifespan for a burst of power... In fact, the real lifespan of a Profound Sage was much shorter than what it was supposed to be on paper.
At present, most of the Profound Sages who were close to the end of their lifespans were those that had used such secret techniques multiple times.