Depends on the mission, for something like this it deorbits fairly quickly due to the drag of the low orbit. To geostationary transfer orbit it takes longer but the periapsis (lowest point) is low enough to slowly deorbit it.

Then we have launches straight to geostationary orbits and similar, for those the second-stages will essentially never come down and they end up positioned in a graveyard orbit.

Even the ISS needs burns now and then to stay up, so yes at that altitude it will fall down eventually.

SpaceX was talking about trying to recover the 2nd stage back in 2018, Elon suggested they would use a giant "party balloon" and land it on a "giant bounce house." I haven't heard any talk of progress made since then.

https://hackaday.com/2018/05/23/spacexs-next-giant-leap-seco...

They scrapped that idea last year afaik and are now focusing all efforts on starship which will be fully reusable.

It's does a deorbit burn to actively fall back to Earth.

Saw a diagram showing that the 2nd stage should deorbit and splashdown somewhere in the Indian Ocean. Not sure when, precisely, though.

It'll do a couple orbits then fall back down to Earth.

burns up.