To draw a realistic face with pencil , how much time you will take for?

IT DEPENDS ON SUBJECT AND ENVIRONMENT MIN 2-3 HRS

It depends on your skills and subject, some newbies artist like me I draw3-4 hours non-stop.

I need 3 hrs to draw a normal Sketch 

It is interesting that you are skilled in traditional art. Drawing a realistic face is quite a fun challenge for many people including me. However, I don't choose to draw realistically as I prefer cartoon designs. For a semi-realistic face, I take minutes as well. But I don't think it turned out good. This is because I'm used to other styles. Hope you keep on drawing