It depends if I'm painting the model to paint the model or painting the model to play the model, but since we're talking "tabletop" quality, I'll go with the latter - since I paint in a fairly relaxed matter, usually sitting at my desk watching some TV show or another on Netflix while painting (right now it's Black Adder

), I'll do a single Fire Warrior in about an hour.
If I'm in a rush, like I was with most of my Boyz, I can do one every fifteen minutes or so, and they still look okay from a few feet away.
I prime my models en masse using Chaos Black spray and try to avoid using a colorscheme that would require another color of basecoat, with the obvious exception of things like Ork skin and Fire Warrior cloth bits. This allows me to simply avoid areas that are going to end up black in the end, cuts down on brushing, all I have to do there is touch up.
On the other hand, if I want to actually put some love into the paintjob, a single model may take up to five hours. I actually spent about ten hours on my Chapter Master, which I never plan to field, just got an idea for a conversion and decided it'd be fun to model and paint.
I have issues with certain details that I can never get right (like faces) so I usually just sort of fudge it, solid color with black dots for eyes. I hate flesh tones. Hate hate hate.