Yes, based on past experience it is roughly every six months for the laptops and iMacs (and often the Mac Mini) ... depending on what advances are made by Intel, NVidia, etc. that Apple thinks are worth using. Usually the cycle is that you get a new spec release, followed by a speed bump release, followed by new spec release, etc.
iPhones and iPods are on a roughly annual release schedule. The Mac Pro, MacBook Air, Apple TV, Airport, Time Capsule, Cinema Displays, etc. are on more irregular schedules.
BUT Apple has moved away from the big shows and more predictable release schedules recently. The rumour is of course that new laptops, probably only MacBook Pros, will be released next week along with the new tablet (and new MacBook Air, new iPod Touches and new iPhones, and new ...), but we'll have to wait and see.