Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Almost every 2nd beta for a major release has come out 2 weeks after the first beta give or take a day or two. It looks like the only exception was iOS 10. I’m definitely expecting to see it today or tomorrow.
 
If you want to be really optimistic, the second beta of iOS 7 and iOS 6 were released 14 days after beta 1. But those were the last times it happened. 8 and 9 were 15 days.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.