From what I can see on Walmart's site ( »
www.walmart.com/ip/Email ··· ethod=rr ), it looks like you can choose the "Go Big" option and get 4GB that doesn't expire if you use it at least once in a 12 month timespan, or the "Go Bigger" plan and get 8GB of data that expires in 30 days. Either option costs $45.
They also have a "Go Big" never expiring 1.5GB/"Go Bigger" 30 day expiring 3GB option for $25.