Historically, the Powerbooks and MBP's used to be updated roughly every 6 months. I think the 215 average update cycle on the buyer's guide takes into account the last several years, but recently it has been taking longer than (9-10 months was the last duration). So the next update could be Jan-Mar.
If you have a good/adequate machine at the moment, I would just wait. Otherwise, it is worth it to get a new one on your edu discount. I did consider the 15" MBP before my edu discount ran out (I used to work for a university), but realized I was mostly lured by the discount, didn't really need a new computer at that time and could wait. In the UK, you are allowed to use your edu discount on one laptop per academic year so if you got one now, you couldn't use the same discount on the next update. So do check the US edu rules and keep this in mind.