How long for doorbell pro to boot up?

How long should it take for the doorbell pro to enter setup mode once plugged in. I have a new unit and connected it up with a plug-in adapter. It's been plugged I for hours and still hasn't enter setup mode. I've tried powering back off and pressing the button on the side but this has made no difference. Have a solid white light, then it goes to a slow flashing then back to solid. This is a constant cycle.