It comes on with the position lights, hence, I turn it on when I get to the "LIGHTS - as required" item on my checklist. I believe that's after the "AVIONICS - on" item but now that I mention it, I'm not 100% sure.
As I understand it, any ADS-B transponder is required to be on at any time that the plane is moving. From that I think it is clear that it should be on before the plane begins taxiing and remain on until it reaches destination and is finished taxiing.