Usually around 45 minutes to an hour, the set procedures I have for creating a decent face take time, and some portions (particularly mouths) take a lot of care to get right.
If the face cooperates, I'll continue it into final stages, and this might take about 45 minutes or so, if you count the extra few times I send him to visit the plastic surgeon in game to correct things don't like about the face. If the face ends up wonky, I'll usually reset the process and try again if I can't correct the problem.
I don't really strive for model-quality faces, but ones that can be easily taken to because of the various imperfections (or rather, human qualities). In short, I try to inject individuality and personality into the faces I make. My avatar (which, coincidentally, is the same guy in 'Rosa's screenie) is what happens when I get it right.
It takes me a similar amount of time to get my PCs look the way I want, solely in the prison cell. I don't go back into the config menus at the sewer gate.
I know the look I am trying to achieve for my Khajiit PC. Eyebrows furrowed in irritation, and a sneering expression on the muzzle. Setting the eyes and brows is easy. Getting the nose and mouth right takes the time due to individual sliders not being able to be locked.
Change one slider and it alters 4 others, including one you only just got right... GRRRRRR!
And once the shaping of the face is done, then I start working on the colours of the fur...
Jenifur Charne