We limit event triggers to one every half hour. The reason for this is to avoid a situation where a user is walking along the perimeter of a geofence and starts sending back dozens of event triggers (think of some users that live along a geofence, they could be constantly entering/exiting geofences racking up data points).
If you have unique needs around the timing of this limit, please talk to your account manager. However, our testing has shown that this time frame of a half hour is the best balance in terms of accuracy and datapoint usage.
To get a better background on Places & geofences read up on it here as well as more information on Messaging with Places.