I've seen it happen one a week as well. It must be on a timer. I was into scanning for 2.5 hours when it reverted to standard anomaly in the sector space.
Yep, it's on a timer. You can actually find out when the timer will be up pretty easily, too. Press J, and scroll forward through the event bar at the bottom. Look for Multiphasic Event. It'll tell you how long until the next one, and it'll be in the center of the list by default when it's in progress.