Forever.
Okay, maybe not forever, a good 20-30 minutes though.
I've made tons of faces, probably about 10x more than characters I've played for more than 10 minutes, which is about 20.
I'm used it it by now, I know what looks good, what does what, and what changes what. If you make a character have an insanely large forehead, their skin becomes baby-soft (but it's not worth it if their eyes move apart abnormally and their faces deform), if you add a beard it changes the body color (mostly affected by Beard Light/Dark and Beard Cheeks), if you add eyeliner a character's skin becomes more yellow, if you make a character have a "Beard Circle" that's dark, then their Lips are going to turn an odd pale color that's hard to fix (which is why I made http://i888.photobucket.com/albums/ac87/MartutTES/ArthurReturningHome.jpg http://i888.photobucket.com/albums/ac87/MartutTES/ArthurTheKnight2.png facial hair instead)
But yeah, it depends on the race.
- Khajiit/Argonian minimal amount of time. Changing hair, maybe a few other things, but mostly leaving it alone.
- Orc takes a little bit longer, maybe 6-7 minutes.
- Humans average at about 15 minutes.
- Dunmer are about 20 minutes.
- High Elves are about 25 minutes.
- Wood Elven males are about 15 minutes.
- Wood Elven females... are at least half an hour if not more.