Slash says that he used to spend 12 hrs a day with the guitar in his teenage , but I'm sure it wasn't daily .

I usualy play guitar not less than half an hr a day (I never force myself to play) , and sometimes when rehearsing for a gig or so I spend 10+ hrs with the guitar per day (practicing alone + jamming) for a couple of days in a row , but not a lot more.

IMHO ,too much of anything and it becomes BORING !