I have kept many close to a decade, volitans over a decade, you can find info in my threads and see many that I have documented through the years. In the wild, lions are reported to live between 10-15 years, the smaller varieties would be at the lower end, you never really know how old they are when you get them. So 7-10 years would be a good goal and reasonable expectation. In my threads I not only include diet recommendations but also info on observation and treatment if necessary. You'll see the current pride of lions I have now that I grew up from very small juveniles to close to full grown adults, my current pride is about 3 years in.
When doing research ask the source for info on how long they kept them and pictures. When you see a 4" fuzzy, regardless of what size that fuzzy was collected at, they have barely been in captivity for a year. They grow very fast to 4", and will be full grown, robust and over 6" about 2-3 years in. I've seen many sources through the years post beautiful pictures and tell people how to take care lions. But the one thing I always notice, there are no full grown adult lions, there's always pictures of new addition.