Seems like the schedule is 3 days of beta with 11-day intervals.
here's a direct quote [copy and paste] of the news from aiononline.com :
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Upcoming Closed Beta Events
Closed Beta Event #3 - July 2-July 6
Focus: Asmodian levels 1-20
Closed Beta Event #4 - July 17-July 20
Focus: Elyos and Asmodian levels 1-25
Closed Beta Event #5 - July 31-August 3
Focus: TBD
Closed Beta Event #6 - August 14-August 17
Focus: TBD
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Have fun all! lots of events to play
Event 4 seems to be the first taste of abyss !