Honestly, I couldn't give you a solid time frame that they might go out on you. It all depends on the environment that they are in and how you use em'. Since you have four Tumults, I can safely say that you aren't over driving them often. Or maybe you do... I dunno.
Modern foam surrounds are pretty robust nowadays. As long as they aren't in extreme conditions, they should last for years. The foam on my Tempest (the original) looks no different from the day I got it and it is about 6 years old. That and I have pounded the crap out of it too.
I'd say you are safe for at least another good 5-10 years. Guess that means the rubber surrounds on my RLp's should last til I die.