It really varies because every pic is different. The average is about 4 / 5 hours because  I take care of every little pixel / detail in the high resolution files, but it can even take me 10, 12…even days. Those things can’t be appreciated at all in the small files you see on your screens, but when you give your product to the model, she should receive a high quality file she can print in a big size.
However, my pics are my “creatures”, they are alive because it’s very usual that I find things I dislike and I like to improve when I look back at them… especially at my older stuff.