WWDC was June 6th, and 13 weeks later is September 5th, with iOS 5 expected to release sometime in September.
So, if we're looking at 4 betas prior to release, and using the first week of September as a "ballpark figure", you're looking at just over 3 weeks between betas.
So, unless the next beta appears in the middle of week commencing June 27th, I've just rabbled on a complete load of nonsense, lol.