SEHunter, the foam in my lube cans life span depended on the quality of the foam itself. Most foam used lasted a couple thousand rounds of tumbling. Once or twice it was pretty soft stuff and started going to shreds in a hundred or two cases used; about four 50-round loads.
The case lube used may also effect the foam's life. All I ever used was STP/Hoppies 60/40. Other lubes may have something that attacks the foam.