You do need to be serious and work on your start up full time, but you will most likely be able to get just as much done working for 60 hours as you do working 100.
The reason for that is because you can actually stay focused on your work for 60 hours. Don't get me wrong if you actually have something to do, its perfectly fine working for 100 hours. But don't try to force yourself into working a 100 hour week, just to convince yourself that you are taking it seriously.