How much data you need really depends on how you use your phone when you’re out and about, but a data allowance between 2GB and 4GB is suitable for the majority of people.
Downloading apps, watching YouTube and using Skype or Facetime will eat through your data pretty quickly.
If you save these tasks for when your phone has a WiFi connection, your data usage will be significantly reduced.
Automatic updates are also something to watch out for. Your phone can download large files without warning when it automatically keeps apps and the operating system up to date. If you’re not careful, these updates can take up a sizeable chunk of your data.
Depending on your phone, you may be able to tweak the data usage of specific apps to make sure large files are only downloaded when you’re connected to WiFi.