How long does it REALLY take to put navigation item on a theme and build pages?

Hi all

I have had varied results here. Some developers will take 4 hours and some will take 2 hours.  Since we are charging client per hour and have to account for each task what is with the variation on this. So to be specific, a PC says on a task, Place Nav Items in Top and Build out corresponding pages.

We need to set a standard time for this as anything over 2/3  hours seems too long. I need developer input of course.

thanks,

juliette