I used to a lot but now only do it if I really want to get an achievement or something.

It often falls somewhere between 3-5 years. The 3 games that are on my list to replay at some point are Skyrim, Fallout 3, and Mass Effect 2. Though I have such a massive backlog of games atm... I doubt I'll reach those for a while.

I've probably gone all the way through Sonic 2... Somewhere between 12-15 times. Sonic CD, at least 10...  That doesn't include the other random times I've played it. Those aren't very long games though. The Zelda Oracle series rings in at 3 completions a piece, and I've been through Mass Effect, Oblivion, and Paper Mario: TTYD twice.... That's honestly it in terms of full replays.