Ah the big question. The truth is it's hard to say. There are a limited number of seats on the bus and it seems like all students intuitively buy at the exact same time.
To keep prices as low as possible our goal is to sell out the bus and in the past that has almost always been the case. To encourage people to plan ahead more we increase the ticket prices two weeks before the departure date and one week before the departure date.
If you're on the fence about buying a ticket our recommendation is to put your order in as soon as possible. If you need a refund you can request one up to 7 days before the departure date. The $20 refund fee is a much easier pill to swallow than a $450 last minute plane ticket (+ cab fare + baggage fees!).