Originally Posted by hoppyhoppyhippo
Depends, you may risk ruining primary fermenter. I would make sure to give it a great cleaning afterwords, if you see any scratches or stains, throw it out.
Why would leaving the beer in primary a couple extra weeks ruin the fermenter? I have several plastic pail fermenters that I've used over a half a dozen times each with no infections. Just don't use abrasives to clean them.
As for the original question. It's supposedly not "good practice" for a lager, but I've done it before (6 weeks actually), with no obvious ill effects.