In detail
100 GB covers about 33 hours of HD video or 14 hours of 4K per month — more than an hour of HD video every day. Heavy social media and music use barely dent it. It's mostly hotspot use, cloud gaming and replacing home internet that push people past 100 GB.
What uses data is almost always video. In HD, 100 GB lasts about 33.3 hours; in SD about 142.9 hours; in 4K only about 14.3 hours. Everything else is light by comparison: 100 GB is about 667 hours of music streaming, and messaging, email and maps use only a few hundred megabytes a month.
As home internet for one person, 100 GB is workable with SD streaming; a household streaming HD every evening typically needs 200 GB or more, or an unlimited plan.
Spread over a month, 100 GB is about 3.3 GB per day; over two weeks it is about 7.1 GB per day. The calculator above shows how many days 100 GB lasts with your own mix of activities, and whether it's enough for a week, two weeks or a whole month.
To make 100 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.