Ive left an ale or two on the yeast cake for months. I wouldn't do a lager like that though just because of the d-rest and lagering, however haven't made many lagers so don't know to much. I usually leave most of my beers primary for 4 weeks minimum. Sometimes life gets crazy and dont have time to bottle and it sits a while. If you do an apa/ipa/wheat it may not be as fresh/hoppy, but if you do something along the lines of porter, stout, belgian, something that usually takes a few extra weeks to come into prime you should be ok as long as temp was fine