I usually never replay them. I'd rather just move on to another RPG. I'm kind of the same way with books as I am with RPGs. Once I know the story, I am not as interested in re-reading it (or in the case of the RPG, replaying it)
I'll replay a game if I'm in the mood or I have nothing better to do. Most of the time, I usually go back because I ignore certain quests or hell, I just play one part of a game and ignore the other paths. Once I beat a game fully, I almost never go back unless I'm hit with nostalgia.
Really depends on the RPG and how many hours I've put into it. For example Fallout New Vegas I spent almost 300 hours on the first profile so I didn't have too much to explore the second time I played it. Some of the old RPGs like Fallout 2 and Planescape offer so much content that I just play them whenever I feel like playing them, without the fear of getting bored because I've already done something.
I usually wait the time when I kind forget all the main things I've done in that game and the storyline too. When I feel like I forget too much about it I reply it but I only do it if the game is really worth it.
It honestly depends on the game. I've restarted some of the Kingdom Hearts games after finishing, for example. But I rarely restart Pokemon games, as I hate to lose everything I've earned.
I never delete my games. I just put them in my storage backburner for nostalgia's sake I usually play them when my internet connection stops or when my friends play it again. I have Ragnarok, GTA, Pokemon etc. I'm waiting for an RPG Renaissance.
Oh pretty much forever, I really need to wait a long time so it isn;t fresh and I just remember it all. Also I only replay the very best RPGS.