I have ordered around 16 extract batches from Northern Brewer. I've noticed a trend in the time the instructions ask you to wait before bottling: if the OG is greater than 1.050, bottle after four weeks, and less than 10-50, bottle after two weeks.
For the higher OG ones, the instructions seem to always call for a secondary fermentation. I have stopped doing secondaries on the suggestion of this stackexchange.
Is there any particular reason for doing 4 week long primaries instead of two week long primaries?
I just made two batches from MoreBeer (my LHBS) and I was told that there should never be a reason to do most ales for longer than 2 weeks before bottling.