I believe the device will check to see if there are updated policies based on the report frequency, defined in the mobile management server rollout -> agent. However, it will only download changes everytime the policy changes. This is an important distinction, as everytime a policy is changed, ALL profiles in that policy are first removed, then all are re-applied. The importance of this is that if you have a wireless profile that is getting removed, and the device is connected over that wifi (and 3G is not avialable), then you will sever the communication with the device until you manually re-connect to a network.