A lot of this depends on style and yeast.
For example, we're just starting to drink a Mild that was in primary for 13 days and bottled without secondary. Used Windsor yeast, which according to Danstar is "non-flocculant", but this beer is as clear as any beer I've ever brewed. It's been in bottles for only a week so the hops are still a bit much.
In this case we made the descision not to use a secondary, since we actually wanted a bit more suspended yeast than we got (not like a hefe or anything, though, just a bit). We did use Irish Moss though.
In general, I'd say that if you don't have a secondary, leaving a beer in primary for 1 to 3 weeks is fine, and for any style where you really want clarity, 2 to 3 weeks is probably preferable. That said, a secondary will help it clear out a bit better and free up your primary for the all-important next batch!