About a week as far as I recall, the supplier is used to international orders. Read the on-line instructions. There is a warning that the mastic on the inside of the door seam might have to be knocked back with a special tool. I made one from wood, but it wasn't necessary on my car.
I also bought and fitted the door guards in matching colour from the US, Subaru UK only offer black ones, which look naff in my opinion. Oh, and also 20mm rear anti-roll bar (sway bar in US lingo) to reduce the terminal understeer!