As you can see, if you buy 10 or 20 cobs then quite a few of them will probably go the distance. My cobs are all three years old and going strong. Take care of them and they will last for ages - like anything else.
There will always be a few duds, though, but you're likely to encounter problems right away if the cob is defective. If they are good smokers from the start, then they will probably remain that way unless some accident befalls them.
As was stated, though, since they are so cheap you won't really be out a lot of coin if they start to swell up or you poke a hole in the bottom or the walls burn out. For a $5-10 pipe they can't be beaten.