Depends on if you really enjoy what you are doing and find it stimulating enough. Being able to take a power nap at times also helps.

I used to work 12-14hrs/day and be highly productive (80/20 rule) as once tasks got boring or my mind started to wonder I would nap for half hour before doing a completly different task e..g. from coding to creating designs in photoshop, which was different enough to use different parts of my mind and thought processes. A change is as good as a holiday.

Most projects were completed as quickly as I could do and because they did not drag-on and because I was able to make lots of headway (reward in itself) I could sustain this for several weeks at a time.

Afterwards I would always take a some time off though :)