>>753
>"monsters live longer than humans"
Generally yes, but it's flexible.
Anyway, in my story, she is about as old as Undyne but not nearly as old as Papyrus and Sans (who are only slightly younger than Asriel and Chara... depending how you look at it) and certainly nowhere close to Asgore and Toriel. Chara's an unreliable narrator and the actual game doesn't give many hints. I leave the reader free to guess how long his ghost floats around, as he can't tell time in that state.
In the game canon, Alphys has to be at least old enough to have been around when the first of the six humans fell in, because she was the one who collected their SOULs into the containers. I can't see Asgore having the scientific ability to do that, even using equipment Gaster left behind. But it could have been hundreds of years between the first human falling in and the sixth one, or it could have been only a few years. We don't know.
I once read a convincing fan-theory that the fallen humans were from vastly different time periods (like the cowboy kid was from the 1850's), so in fact centuries passed, but upon further reflection, it just doesn't add up. It seems more to me that they were all just neglected children who got their personal items from the Surface trash dump that flows into the Underground trash dump. A torn notebook, a burnt pan, a dirty tutu... just things other people threw out that only children with active imaginations could find value in.
There's a reason why I'm sure Gaster was gone before or soon after Chara came along, but for the life of me I can't remember exactly what it is. Of course, I could be wrong. Undertale is fraught with plot holes and assumptions, and it takes a lot of mortar to repair the gaps in the wall, so to speak. We're all left to mostly wild-guessing.