I fill it up for 10-15 seconds then stop for a few to let air out of the tank..
When you say "letting air out" what's really going on is that the air above the fuel in the left hand tank is venting to atmosphere through the vent hose in the upper part of the filler neck as the fuel is 'self balancing' between tanks via the largish hose near the bottom front of the tanks. Which is why if you just pump fuel until you get a (single) auto cut off the tanks won't be completely full as they haven't had time to balance between the two. Not all bad as the issues with over full tanks are well documented here.
