You should contact your cellular service provider for specifics. But on my iPad I have a 1 GB per month service plan. On the 12th of each month my credit card is charged $20 and my plan is refreshed to 1 GB. The amount not used from the previous month is not carried over. If I approach my limit in a given month I will be notified that my data is almost used up and I'm given options to increase my plan or to purchase a new month. The plan does not expire. If I don't want cellular for a period of time I may cancel the service.