In detail
30 GB is a comfortable middle ground: around 10 hours of HD video, about 40 hours of social media video or hundreds of hours of music per month. Most people who use Wi-Fi at home never reach it.
What uses data is almost always video. In HD, 30 GB lasts about 10 hours; in SD about 42.9 hours; in 4K only about 4.3 hours. Everything else is light by comparison: 30 GB is about 200 hours of music streaming, and messaging, email and maps use only a few hundred megabytes a month.
For a month of travel without reliable Wi-Fi, 30 GB is a sensible minimum if you also want to use video calls to stay in touch.
Spread over a month, 30 GB is about 1 GB per day; over two weeks it is about 2.1 GB per day. The calculator above shows how many days 30 GB lasts with your own mix of activities, and whether it's enough for a week, two weeks or a whole month.
To make 30 GB last longer, set video apps to data saver on mobile networks, download shows and playlists over Wi-Fi, turn off autoplay on social media and check which apps use background data in your phone's settings.