Because most of the Egg... the base, lid, inside firebox, and fire ring are all made of almost 1" thick ceramic (not ceramic like a coffee cup) it will never rust out. However, it is possible to crack and it is covered for that for a lifetime. Mine actually developed a crack last year for the first time. I think it was from the time we had a tornado and it got tossed across the yard but it didn't show up until last year. It doesn't affect the cooking but I have a free new base (bottom half of the Egg) sitting in my garage for whenever it does decide to break or I decide to go ahead and make it a flower pot.
The metal ware (bands and hinge) are not lifetime warranty. They are made of powder-coated steel and will eventually rust. But they do last a very long time (at least 12 years) and are not too expensive to replace. They don't use stainless because it expands more with heat. It would be risky to the heavy lid if the band loosened up.
I can't remember that my Egg sits outside in the weather. Sometimes I cover it but most of the time not. It can be 15 deg outside and I will fire it up and have it at 600 in a matter of minutes. Because of the type of ceramic, it doesn't hurt a thing.