your max overclock for 24/7 use is the maximum you can reach without getting artiffects.
artiffects are miss calculations done by ur gpu and can be spottet in games most of the time the show as wierd blocks or collors.
you can als get gpu stress tests wich can spot artiffacts.
when you'r card is artiffecting you have to turn the clock down till it stops and thats the max your gpu can do.
BUT dont forget to watch your temps keep them below 85c I would say..
also ur PSU has to be good enough too handle the extra amps