As long as they're clean and you're sure there's no rot you should be OK for a year or two at least. There are two potential problems. The first is blight, which overwinters on the tubers. It's definitely a problem; seed potatoes are a souce of blight in the States, though I've never heard anything of the sort over here. I suspect their standards are a bit lax. The second is viruses. These accumulate from one year to the next, and eventually the quality is likely to deteriorate.