How quickly does it generally take for American to get used to driving in UK?

I was in French New Caledonia as a kid, sitting in a bus behind the driver. As we drove along the mountain road, I can see a car coming towards us. Coming closer. And the bus driver is pulling over to the wrong side of the road.. And he’s going further over, and … fortunately the car coming towards us sees that our driver is an idiot, and passes us on the wrong side of the road! I take a breath and see another car coming towards us. Our driver is veering to the wrong side of the road again! No way, I think, that this car is also going to be smart enough to pull over to the wrong side of the road to let us pass.

I’m just resigning myself to my fate when the penny drops.