No pattern for me. Rebooted once just an hour after finishing the update, wasn't running anything - was just sitting in my cupholder. Hadn't even unlocked the phone following the final reboot. Second was the same night, again just sitting there in the cupholder.
Third time was in the gas station, setting up GPS Navigation when I received a text message - went to check the text message and reboot.
Can't even remember the other ones but the most recent -- was in the car using GPS Navigation on the charger when it rebooted. Phone GPS may lock faster, but phone is not reliable as a GPS navigation anymore since I can easily miss my turn in the 3-5 minutes it takes for the phone to reboot.