As others have said, it definitely depends on the game:
if I’m not enjoying a game, of course, I can be done with it long before it’s finished;
but I also increasingly find that if a game continues on for too long, I’ll drift away from it if I take too long a break between sessions (I’ve never finished Opus Magnum, for example, despite enjoying it, because I just didn’t get around to coming back to it). I’m not sure if I’m “done” with these games completely - there’s the possibility that I might come back to them - but in practice, I rarely do. The period of time this takes depends on how much I’m actually enjoying the game, and also on how busy I am with other stuff in the same period.
for almost all games which I complete, I’ve been done with them basically as soon as I get to the end of the “narrative”/“You Win” screen.
there’s a minority of games which I’ve liked enough, or have been short enough [the shorter a complete replay is, the less I need to have liked it to do this] for multiple playthroughs soon after the first run through. Usually I’ll feel done before I finish a second pass though (I got about 2/3 of the way through Transistor on NG+ before stopping, for example), and I think it’s only Analogue: A Hate Story and its sequel Hate+ which I’ve actually exhausted all the possible endings for.
(There’s also the separate category of “nostalgia games” which, like old books, I might pick back up briefly for replays, or partial plays, of long after the time I first played them. Tellingly, most of these are from more than a decade ago - in fact, apart from Spelunky and Super Hexagon, almost all of them date from before 2000 - so this is a different kind of thing I think, more of a “formative experience” being returned to. I’m not even sure if Super Hexagon counts as this - it’s arcadey enough that it might just be a good flow game…)
As I get older, more games fit into the first two categories for me than any other. Like keydemographics noted, lots of modern games seem to be increasingly full of “Content for the Sake of It”, and that means that they tend to outstay their welcome, for me.