More dependant on how much sand you pack.
Too dense it'll split open.
Working "hard," or the perception of working hard, doesn't really mean anything. Sweating, vomiting, and breathing hard could be a good workout or a tropical disease kicking in.-Dan John
from MY experience, since i HAD one. it broke, but it shouldn't.
had 40kg inside, NO PROBLEMS!. added 20kg into the bag, but i didn't give the sand enough room to move, so when i cleaned it and dropped it. BOOM. haha. needa get a new bag too.