There's no set frequency as it really depends what we're working on at the time. A good rule of thumb is to look at how often our updates have been released in the past. We normally have at least one, and usually at least 2-3 testing periods per update.
Also, I will receive an email if I'm accepted and it's time to test? If so, then I'll have another reason to check my email.
Yep, you'll receive an email each time a test is available if you've been accepted. Is that why you've applied? But really, as mentioned in the original post this is mainly testing for bug fixes and other minor things.