potatoes have only one crop as you need to pull the plant up to get to the pots. you will know when ready as the flowers will die and the stems start to wilt and flop over. my grandad used to leave the pots in the ground till he needed them or until the end of november, whichever came first. if you planted it earlier enough you may have many more pots than the one you put in, but if you only planted it recently, then you won't get much from it I am afraid. the easiest way to look though is to carefully push away the soil around the stems and see what you have got but you must replace the soil or any exposed pots will go green.
thanks for that. we've not had any flowers yet, so i guess its going to be a while yet. we had some shoots coming through and I've covered them up with fresh soil. is this correct? all help appreciated.
if it's outside then like Misi says don't expect too much of it - there's not much time left this year. Don't push it too hard, just cover the stems a couple of times and then let it do its thing. It needs to stay damp - keep it watered! shouldn't be any need to feed it if it is in commercial compost.