The logs don't get truncated on the Exchange server until the Recovery Point runs.
What happens is the Recovery Point runs and then the BE media server contacts the Remote Agent on the Exchange server and tells it to tell Exchange to truncate the logs up to that point in time.
All CPS does is replicate the logs so that BE can organize them with the previous Full backup to generate a pseudo Full backup inclusive of the current logs.
So if the Recovery Point runs hourly then the logs should get truncated at those times.
You should be able to look at the time the Recovery Point ran successfully then go to the Exchange server, look in the Application Log and examine the entires from Exchange. It should show that a backup started at the time the Recovery Point ran, another entry will show the range of logs backed up and yet another entry should show the logs were truncated.
I have run this in my lab on a regular basis and it does work. The only time I had a problem was when my CMS was shut down for a day and the Recovery Point failed, once my CMS was running again I re-ran my Exchange Full backup and my Recovery Point backups started to function again.
I'm unaware of a problem related to that process that we are hotfixing. I apologize for my ignorance if there is one, I will have to investigate this possibility and will look to re-post to this thread.