to quote Shade "if you enjoy playing you'll practice" or something like that haha ..
when i discoverd Erik Mongrain and the style of Flat Tapping i was ADDICTED and played / practiced at EVERY opportunity ..
i remember LITERALLY "crawling" to get to the guitar only to decide i better get some sleep instead haha .. that day was a WHOPPING 16 HOURS .. seriously .. i played from the time i woke up till the time i went to bed that day .. haha .. yeah i'm a nut ..
for a few months i was practicing a minimum of 10 hours a day.
I have a few things to say ..
First .. it scientific FACT that the brain operates in 20 min modes .
EVEN IF you play constantly for hours .. mix it up every 20 mins ..
Often i'll reach a point i call "diminishing returns " haha
which means, yeah i'm going through the motions but i'm not gettiing anything out of it .. or making too many "silly" errors.
As EVERY great guitar player will tell you, practice at the speed where you can play it "perfect" then speed it up .. SO IF you reach the point of diminishing returns .. your making mistakes and YOUR ACTUALLY PRACTICING TO MAKE MISTAKES .. i'd rather slow it down to MEGA SLOW speeds and play the peice .. try that some time .. its actually harder than playing it fast .. if you stay in time that is ..
As far as getting tired .. eh .. well there are times when i'm tired and playing guitar actually revitalizes me .. its 90% the cause for my insomnia haha ..
ok .. now my take on the whole carpel tunnel / playing too long thing
I'll start with a quote: "the path of excess leads to great wisdom" haha
never more true ..
CONSIDER THIS .. the amount of force it takes to move a string a few millimeters to engage the neck is a TINY amount of force .. VERY VERY TINY .. so .. WHY DOES IT HURT SO MUCH ??
its because your using excess force. "over gripping"
the best way to find out where and when your over gripping is to play till it hurts .. and LEARN to play relaxed with just the right amount of force.
now i don't mean THRASH till you CRASH .. just play till you feel some effects and you have to make it uncomfortable or you wont reinforce it enough in your brain to NOT DO THAT .. haha.
pain is a great motivator .. it can be helpful and hurtful ..
walk the edge and play safe ..