Since we are on the topic of O2-absorbing caps, I have always wondered about the science behind how these work. I have always seen these things packaged in plain old plastic bags. So the caps are always exposed to oxygen - how can continually absorb O2?
I heard that they have to be wetted to activate them (meaning that you can't sanitize them before using them, if this is correct). Anyways, if this is true, I would love to know the chemistry behind this.
To the OP -- I believe that there is always a small chance that O2 can seep into the bottles, but only for beers that you age for > 6 months (more like on the timespan of years). Regarding how much O2 is needed to spoil a beer depends on the style. Big, robust beers with lots of dark malts have a lot of natural anti-oxidants in them. Pale, thin, delicate beers spoil very quickly with only small amounts of O2. Bottle conditioned beer are also more resistant to O2 because active yeast will consume it; beer that is fined or filtered, then packaged without yeast stale more quickly.