I can do 8 hours of light-weight development easily enough, things like minor bugfixes, minor tweaks, etc. and carry that on for months if needed. But for stuff like coding from scratch, aside from brief bursts, I usually find that 4 hours is a good general guide. Obviously when nearing release, the long stretches of 8-10 hours of coding is going to be much lighter. Most of the tweaks are going to be minor, you hope, and so you can throw the rules out the window for a bit.

If you are working 60-80 hours a week, hopefully you aren't just doing one thing. It's one thing to put in 20 hours a week on software development, 20 hours a week on sales and marketing, 20 hours a week on fundraising, and 20 hours a week on business administration. It is far worse to put in 40 hours a week on software development, IMO.