I've been working 7am - 3pm at my day job, 4pm - 11pm at my side projects. Dropping the day job.
It's not a way to live forever, but for people who can work like that, for a few years it can suit them really well.
The con of the startup racket is that so many people think they're buying their freedom with the 16+ hour days (and have been misled into believing that) when, in fact, the proportion who actually become billionaires or major venture capitalists is pretty terrible.
I'm productive about 8hours in the day, but I put in 10-12 daily.
Overtime I'll start feeling like my managerial duties are productive uses of time. They are, but as a hardcore technologist, it's tough to shake the if !programming then wasting time.