Mine tend to take days, and weeks, and sometimes months simply because I have a hard time finding the time to work. I have to squeeze it in when I can. On top of that, I'm just not a very fast artist. If I could work on one piece from beginning to end in one shot, I'd probably be looking at a couple/few hours (sketchcard) or up to twenty hours or better (painting or pencil portrait), depending on what I'm doing.
Depends on what kind of art, if it's just a character, or a scene with a lot of details and a background... Some artworks take just a couple of hours, others take several days to complete. I used to sit for entire day (8-10 hours) completing one work, but I can't do that anymore because of my job and other things, so spreading the work across several days works better for me in any case. Plus you get a chance to see your work with fresh eyes every day, and spot mistakes you wouldn't if you were looking at it the entire time.