I pretty sure it's posted 1:00pm. On our prior 3 Princes cruises, we've boarded around noon each time. The biggest factor in boarding time is getting clearance on the cruise that just ended. Whatever time that is, they add x amount of time to.
Noontime seems to be the norm for Princess. Also, they have their desks set up by Cabin number which makes embarkation very smooth. If one desk does not have a line, they will motion the next awaiting guest to their desk. Very organized.