Plan pages list gigabytes, which is precise and unhelpful, because nobody thinks in gigabytes on holiday. They think in "will I run out before the taxi finds the hotel." Here's the honest breakdown, habit by habit, so you can pick a number that matches how you'll actually use your phone.
None of this needs to be exact. You're not budgeting a household bill, just avoiding two outcomes: buying far more than you'll use, or running dry on day four with no signal-bar mercy left.
The habits that actually use data
Most travel data goes on four things: maps, messaging, the odd translation lookup, and photos syncing to the cloud. None of these are heavy on their own. Maps with turn-by-turn navigation running for an hour or two a day - getting to the hotel, finding dinner, wandering back - uses roughly 150MB a day for most people, more if you leave satellite view on the whole time, less if you glance at it occasionally rather than following it live.
Messaging apps, including WhatsApp calls, are light. A day of texting, voice notes and the odd video call rarely clears 100MB. Translation apps are lighter still - a handful of megabytes for a day of pointing your camera at menus. Photos are the wildcard: if you've got cloud backup switched on, a day of holiday photos syncing over data rather than Wi-Fi can add several hundred megabytes on its own, invisibly.
Adding it up for a week
Put together - maps, messaging, translation, and modest photo syncing - a fairly typical travel day lands somewhere around 300 to 500MB. Over a week, that's roughly 2 to 3.5GB for someone who's out and about, navigating, messaging home, and not streaming video over mobile data. It's why our smaller weekly plans tend to sit in that range rather than the double-digit gigabytes some plans lead with.