Balancing work and play

Hi, how do you decide how much you should work and how much you should play (rest and/or do fun things)?  I’ve recently started working full-time and I’ve fallen into the trap of overworking to the point that I’m physically and mentally exhausted by the end of the day.  I’ve been dialing it back lately and using Monday Hour One to enforce more time for play – but how do I figure out how much is appropriate?  How many hours is too many to work?  How many is too many to rest?  Is there a module on how I might work this out for myself?