So, this is a simple rumors thread, I'm going to share when I think it will be released and what I'm basing it on, hopefully you will do the same in the comments!
I reed on iH8sn0w's twitter that the iOS 5 beta 3 will expire on the 18th of august 8:00:01 PM EST. My guess is that iOS 5 will come out around the 15th, why not the 18th? All the developers version will expire and they need time to update to the new iOS before the beta expires, othervise they will be stuck with iDevice(s) that they can't use. It might be released in the first few days of September also, if there's another beta, beta 4.
I think this will take more time because Apple always release 4 betas or more.