If you're using a plastic bucket for a secondary you probably shouldn't let it sit for more than four weeks. As joe said, plastic will allow oxygen to pass through.
The textbook reason for using a secondary is to avoid autolysis. That's when the yeast commits suicide after sitting too long without food, and makes your brew taste like a burnt tire. For this to happen you'd need to let the brew sit on the primary yeast cake for a while after primary fermentation is done. I've never had it happen and I've let batches sit in the primary for over a month.
Another purpose of using a secondary is to allow as much yeast to settle so the brew is clear. That can take a few weeks or more. If you don't care about clarity or don't plan to give it at least three weeks, there really isn't much advantage to using a secondary.
If you ever lager it is to your advantage to rack into a secondary and then use that for cold storage.
Anyway, I do use a secondary (glass). Just this past weekend I kegged a batch that I racked back in July. No sign of autolysis, and super clear.