apartment search timing

[somerfriend.livejournal.com]

When do you consider to be the customary time to look for or advertise for an apartment for rent.   3-4 weeks before move in?   Does it vary based on how far away you are from a T stop?  (eg Davis Sq apartment being high demand are a different market)  And whether or not it is Sept 1 or not?   (Sept 1 seems to be a more extended search time?)