For me, it was until I felt tired, so would go to sleep between 1-2am. It was strictly 6 hours/day for me during the weeks leading up to my exams, with 1 or 2 20 min naps during the day.
Seriously, take a nap during the day! For 20 minutes... you'll feel like you have a new brain, and it helps with forming memories and turning your STM (short term memories) into L(long)TMs.
There's an app called Pzizz which puts you to sleep like a baby, and wakes you up shortly after feeling so (SO SO SO!) refreshed and recharged. It's for Android and iOS (as well as Mac and Windows). I'll admit that I was a skeptic and yes, I shamelessly torrented it last year thinking it wouldn't work, but happily paid for it when I realised that it has the power to revolutionise your day and workschedule. Try it!