My personal best, which is a portrait of my deceased grandfather, took 3 months to complete, but i did not work on it for long periods of time...i was letting it "marinate" between sessions...hahaha
So i would have to say that it took perhaps two full days to complete (if i had worked continuously). I'm not a portrait artist so it was a real effort for me...
Surprisingly...i think of it as my best work yet.
For other subjects, i normally finish a pastel in an average 5 hours unless it is much bigger than 16x20.