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Every 2 to 3 weeks. Don't expect the next release to be bug free. Occasionally one beta will be worse or better than the previous.
 
Not sure about anyone else, but I'm thinking that I've already had an Over the Air update last night some time. The reason I say this is because my clock alarm system has gone from having the AM/PM bug in 24 hour mode to actually working in 24 hour mode correctly.

But I could be wrong that could have just fixed itself randomly being a beta and all.
 
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