I think it depends on what i am working on...but when it comes to nice pieces, probably a week and then some. I take forever trying to make things perfect. and seeing as i only have either after work or before work and the weekend to work on something, things can drag. i can spend hours trying to make one bit perfect. i think i take too long. just need to work smarter i guess.
It depends on the piece, really. More detailed ones take longer, simpler ones don't take as long... but roughly a work day? A little more? I guess it also depends on the size of it. I've worked on large pieces in traditional media that I had to work on over the course of days/weeks, but I didn't work on them all day due to other obligations.
Comics work has trained me to think that if I'm spending more than a full day on a single page, I'm working too slow or over-working the piece and probably sapping it of is vitality and dynamism. Not necessarily true of every piece, of course, but I really want to train myself to work fast as a point of professional necessity.
Also, I get impatient and fear I'll never finish a piece if I take too long. I admit, I'm a bit of an attention hound, and I can't wait to post my work online... :/