I've done this a lot. If it's a pretty good game I'm likely to replay it. There some games of which I probably not going to replay even if they are really good like Zelda. A long rpg that's not like KOTOR I'm probably not going to replay either.

I always go back.
I just go and start a new game when I feel like it and just run through it all over again just like the first time.

I do it constantly with 2D Platformers.

I always return to completed games. Sometimes I wait until I have forgotten where items and secrets are and then replay again.

Basically never, which is unfortunate. There are plenty of games that I haven't played for years that I'd love to get back to - unfortunately (fortunately?) I have literally hundreds of unplayed/unfinished games on my shelves. I'd much rather go through and finish those before I replay games.

Having said that, there are definitely games that I want to go back and play again some day. Given that I don't buy many games these days, I should eventually get through the ones I already own. When I get to that point, though, I'll be much more likely to replay shorter games, however, than 40-hour-long epics.