From July 11 to July 16.

It's been more than 3 weeks (23 days) since the last event ended. The longest we've had to wait in a long time. Usually it's less than 2 weeks, and between the last 2 events there were only 8 days.

And the event is only 6 days long (which I don't care much about since it's CoT).

Official News post:
Caverns of Time Event, 7/11 - 7/16