Around 4 days (in and out ) ,i try not to rush it and give myself a day or two to rethink the drawing and see it with fresh eyes . It can be done in a day but i will make mistakes and will regret them latter on and will have to fix them .
Start with block out shapes don't zoom too much and go to detail until the forms are defined . I guess there are many factors that speed up the process (lasso tool to catch you important forms so you don't need to worry about them latter , black and white block out before you go to color , reference , color sketch ,) and experience most of all .
I am very lazy artist so i had to learn to draw fast so that i can keep up with my client demands