As Steve and Bob noted, people will commonly say that Muir is a ~45min side trip. I found the following thread from a few years back helpful with planning, particularly the last summitpost link with the route map drawn over a photo:

http://www.whitneyzone.com/wz/ubbthreads.php/topics/37017/Re:_Combining_Mt._Whitney_and_

Aside - cool to see Steve and Bob still commenting many years later, we appreciate your contributions!