Difficult to say, depends on the height of the tub. Usually top up as the shoots grow upwards to a couple of inches from the top. The idea is to prevent light getting to the tubers so they don't turn green. I usually put four canes inside the tubs and tie string around them to support the potato tops.

Yes keep topping up the tub as the green growth appears, rather as you would ridge up a drill of potatoes. You only stop topping up when the soil is almost at the top of the tub.