Check your actual usage
Both iPhone (Settings › Mobile Service) and Android (Settings › Network › Data usage) show how much mobile data you used. Look at two or three months, not just one.
If you are mostly on Wi-Fi at home and work, your mobile use is probably far lower than headline allowances suggest.
Rough guide
Messaging, email and maps use little data. Social media feeds and music streaming use moderate amounts. Video streaming and hotspotting a laptop use the most.
Light users often stay under 20GB a month; heavy streamers or people who hotspot daily can use 100GB or more.
Don't overpay for headroom
Large allowances are now common and cheap, but compare the price, not just the GB. A plan with 100GB you never use is not better value than a cheaper 50GB plan.
Use price per 10GB on our plan pages to compare finite allowances fairly.