This is a hard question to answer. Many of us spend most of our time within range of a wifi router we have the password for, whether at work or at home. This means we only need a data allowance that covers the times when we’re not, as your phone will automatically use wifi data where it is available. As a rule, then, most of us need less data than we think. 5GB is a good place to start. If you hit the limit in a month, you can always upgrade.