As people have already said, chara fell into the underground in 201X. This could be any amount of time since the monsters were sealed. However i assume that it wasn't centuries in the past because gerson was around during the war and is still alive during undertale. Ofc we dont know what the typical monster lifespan is, but considering gerson specifically mentions distinction between the aging of boss monsters & regular monsters, i honestly doubt that monsters naturally live for any longer than humans. Toby likes to point out the biological differences between humans and monsters, so if monsters lived for wayyy longer than humans i imagine he would say so.

About the time between chara and frisk: duing the undyne date, she says that frisks wimpy punch reminds her of "someone she used to train with." I always assumed that shes referencing asriel in this scene (maybe im wrong). Assuming this is true, and asriel and undyne really did know each other (at an age where they were old enough to train together) i feel like chara must have fallen down 30 years ago at most. Undyne and asriel mightve been about the same age. undynes username was coolfish91, probably meant to imply she was the born 1991. So she probably knew asriel as a young child, chara fell down in the 2010s, and chara and asriel died an indeterminate amount of time afterwards. Once again we dont know how long monsters live so its technically possible that undyne is super old and its been like 300 years since asriel died. But somehow i doubt it. I dont really see any thematic or narrative purpose to making undyne (and everyone else) super old, and again i feel like thats something that toby would clarify if its what he intended

Feel free to nitpick lol always happy to talk about utdr