It seems to take me absolutely forever. At least a few days, usually a week or so. I RP all my characters to a degree, and it tends to take me so long to think of one that's believable. And then there's choosing all his skills and specializations. I find it to be a problem when I haven't played Oblivion for a long time, and when I get the itch to visit Cyrodiil again with a new character, it takes me so long to think one up! I just want to play, but I don't want to play without a nicely fleshed out character. It's a conundrum

How long does it take you guys?

Takes me forever too....

They pop up in my head all the time. Not fully fleshed out perhaps, but not just vague ideas either. The rest then comes from looking up mods that go well with an idea, the initial building and final touches are done while playing and thinking how this character would approach a situation in game. I also often have ideas from watching TV, reading or playing other games.
It's annoying, because I always want to start over with a new character before coming really far with the current one.