The frequency with which a device refreshes its location is dependent on several factors, the most important of which is battery life. As battery life decreases, the device will refresh its location less frequently.
We've found that in most cases, devices tend to refresh their location at least once every minute. At the long end of the spectrum (when a device has very low battery), it can take up to five minutes for a device to refresh its location and some devices actually disable location updates when battery life becomes critical.
To get a better background on Places & geofences read up on it here as well as more information on Messaging with Places.