As mentioned above it varies largely based on game structure and the size of the field. Some games can take 4-6 hours start to finish while some of the marathon tournaments on ACR can run for 15+ hours. I've played quite a few ACR tournaments for 12 hours only to bust in the top 25.
This is all about your schedule and good game selection - I never play the marathons unless I'm ready for such a thing and have the time to do so. When I play week nights - I work a regular job during the day, so I don't register tournaments that will have me awake at 2 am or 3 am.