Anecdotal evidence from my own experience if anyone's interested.

I was waking up at 10-11 AM worked through the day without breaks until 5-6 PM, had dinner with my better half, watched some TV, met with friends, etc and then typically resumed working at around 10 PM for another 4-5 hours. That works out to about 11-12 hours per day. Plus another 10-12 hours per weekend -> a bit over 70 hours per week.

I was going at this pace for close to 2 years. The work included designing and coding the product (windows/linux/osx clients, the backend server and the web interface), graphic design (incl. website), system administration and support of a live server cluster, support (ouch), busdev, etc .. pretty much everything except for the accounting and legal/IP stuff.

Stressful, but totally doable. YMMV of course.