It depends on how you store and use them. For one, they don't hold as much pressure as a corny (max 10-15 PSI I believe), so you have to be careful not to bulge them. Second, depending on how much moisture is in the air where you store them, you're likely to see some rust on the seams. The last, and probably most important, is cleaning the inside. Since they're a softer metal, make sure you use a good soft brush to get all the nooks and crannies without causing scratches that will promote bacterial growth. I believe they have some kind of coating on the interior that wears down after so long, not sure though.
I say use them till they get rusty, misshaped, or the beer taste funny (They're not overly expensive).