I usually buy 2 kilos at a time (4.5 ish pounds?) but there are only two of us and we don't eat potatoes that often, so it usually takes us a couple weeks at least to get through the big bag. For me they stay fresh for about a month in a paper bag in the produce drawer of the fridge. I think the important things are that they are kept cool and away from light.
It helps if you can see when the potatoes were harvested (obviously the more recently the better). If you buy potatoes in bags they often have the packing date somewhere close to the best by date.
I think potatoes freeze decently. At least I have bought pre-sliced frozen potatoes before. It could be that they were partially cooked before being frozen though, as that is usually what is done with frozen veggies. They will definitely be ok cooked in something and then frozen for later use.