My experience has been as long a you don't let them get dry, they'll last you a couple of week's without issue (if not longer). Of course you'll have one or two here and there that go south quickly, but at the rate of 1 - 2 dollars a blank carto that is acceptable in my book.
Where are you getting them for 1-2 $ each? My 510 will be here tomorrow and i wanna try a carto.
thanks