I think you get a monthly allowance - same as free minutes on your mobile - and if you don't use the allowance up by the end of the month you lose it. You can get some that are only on a month's contract, which you can renew every month till you decide you no longer want it, and others which have 12/18 month contracts. If you would only be using it for going away you'd be as well to just get the 30 day one rather than commit yourself to a lengthy contract for something you may not use.