After I get the Auth token from https://www.google.com/accounts/ClientLogin do I need to send 1 message per registration_id, or can I post a message to many (1:N) registration_ids at once?
If I have to pass 1 post per registration_id (1:1) , how often do I need to refresh my Auth token before it becomes invalid
It has to be 1 message per registration_id. Currently (June 2012), C2DM does not support sending notifications in batch.
The auth token should be good for a long time, in my experience, it's good for days, if not weeks. Of course, you can check the send response for 401 code, and redo the authentication.
Update 6/27/2012: Google today announced the deprecation of C2DM and replaced it with GCM. It looks like GCM allows sending notifications to multiple devices in one HTTP call. See here: http://developer.android.com/guide/google/gcm/gcm.html#send-msg