They run, from the airport, when they are full.
In our case this meant waiting for 15 minutes for the next shuttle to our destination (anaheim) to arrive, then a complete lap of all the terminals til the bus was filled - which took another 15 minutes.
But it worked well, only stoopoed at 3 other hotels before ours, and got us from the airport to Anaheim in about 90 minutes including the wait time.
For leaving you are supposed to book a half hour window - I booked 7:45-8:15 but had a call from the front desk at 7:30 to say they were waiting for me.