potatoes-earthing up question

have some potatoes growing in a bag and also on the allotment but have a question about earthing up. How often should it be done and how many times? when can they just be left to their own devices? the ones in the bag on pation came up then i put some more soil in and now they are coming through that. ones in the lottie have been covered over once.

It's not so much a question of frost, more of keeping the light from the spuds or they go green.

I do it about 3 times during the season, so that ultimately the ridges reach a height of 6 - 8".  Ridging up in stages helps to keep the weeds down as well.

If you are not expecting frost, do NOT cover all the leaves as these are essential for the plant to make food.