The update frequencies for sending XMPP packets into talk.google.com as for Google Talk and AppEngine XMPP applications are determined by a throttle.
As of this writing, maximal message throughput frequency is once in ten seconds. This frequency is determined over ten messages by an averaging calculation.
Exceeding this frequency will result in an XMPP CANCEL error which is not recoverable. The CANCEL event should be followed by a disconnection, and then a fifty second pause before reconnecting.
As XMPP is the continuous stream for AppEngine, it's a nice fit for a number of applications.