Plastic fermenters aren't all that o2 permiable. I've had beers in them for a couple months with no oxygenation or other ill effects. So in my experiences,it diesn't happen very fast if at all. contrary to all these "scientific" findings. but if you're going to age for 6-8 months or more,then glass is def benificial. I even spoke to the head brew tech at Cooper's about this & he said even their PET bottles are good for up to 8 months in terms of o2 leaching. But he did also say that their PET bottles have a nylon coating inside to combat this. Otherwise,fermenters are about the same in terms of the plastic. A month or two,fine. Long term,no.