Every 2 years - learned that from experience
I am ready to do my third refresh on a 2-year cycle. I have found that much beyond 2 years, you start running into issues.
I have only one computer, my MBP, and I use it for work (Photoshop, Illustrator, etc) and play (Aperture with large Raw files; low-end video editing). It gets used about 8 to 10 hours a day, and after 2 years, its pretty well beat up!! I calculate well over 6,000 hours of use, and so even with the high end 17" that I have grown to like, that works out to be around $0.60 an hour...
This last time I purchased a refurb 17" rather than the new 15" I got back in '06, and that worked out OK. Its battery just burst, and I got a replacement, and it has gone through a period of instability, which may be a predictor of imminent hardware failure. I'm not worried - backed up in the cloud via ZumoDrive - but ready to trade up.
I geekbenched mine, and it came in around 3500 - so if the leak at 5260 is right and they put in the i7 I am about to experience a significant boost in speed! I have been in the apple store a few times, and noodled around with the large files in Aperture that they have on the current crop of MBPs, and it is super fast - no endless beach balling when you clone or repair. You could say that upgrading my camera this past year, and so nearly doubling the size of the Raw files, is making this upgrade very timely.
So I hope for tomorrow, but am ready to wait a few more weeks also...