Not the answer you want, but it really depends (on the beer, way oxygen was introduced, etc.).
My example:
I typically quick-force carb my beers by turning the PSI way up and rolling the keg. This works great, however, I got lazy, and forgot to purge the headspace with CO2, so for awhile, I was diffusing O2 and CO2 into my beers. I realized it when I made an IPA for a vet that was coming home. Sent a few bottles to his house after kegging, which people raved about. Brought a bottle to a club comp, which it won. Then entered another sanctioned comp in NYC, which occurred about a month later. The beer was lucky to have scored in the low 20’s. Cardboard, acetaldehyde, diacetyl, lots of bad things. I actually did have one bottle left over, which I tried, and it sucked.
My understanding is that basically O2 can cause some of the intermediate compounds that are produced during fermentation (like the ones above) to redevelop. So it can really be like a hand grenade going off in the flavor of your beer.
I’ve heard that hoppy beers oxidize easier (something to do with the dissolved oils/acids breaking down).
Also, I’m a believer in the O2-absorbing crowns. I’ve done side-by-sides in beers that have been aged for over a year, and the O2 crowns definitely keep the beer more stable than standard crowns, while still allowing the light oxidation/esterification of alcohols that makes aged beers awesome.
Soooooo, to answer you question, with me force-diffusing about a cubic foot’s worth of CO2/O2 together into the beer, it destabilized pretty quickly within a month. A little bit of stirring or splashing: I probably wouldn’t be concerned about it on a homebrew scale.
