If you don't combine the parts A and B of the silicone, it has a shelf-life of six months after the manufacture date. However, you have to keep it in some relative sort of climate control - you can't just leave it outside in the middle of summer and expect it to be fine, or in the garage in winter and have it be ok. It will go "bad" faster that way.

I've used 7 month old silicone and it was still ok.... But it had been opened only once before, and I kept it inside the house where it was at a constant 23.3-24 C degrees. Generally speaking though, it's not good to use silicone over 6 months old. And to bank on it still being "ok" much longer than its expiration date seems like an expensive gamble IMO.