Kraków, Warsaw and Gdańsk: how much data
Kraków is where most visitors start, and a weekend there, with a day at the Auschwitz-Birkenau memorial or the Wieliczka salt mine, needs nothing bigger than 3 GB. That pack lasts 15 days, and most of it goes on entry times, transport and the map of the old town.
Link Warsaw, Kraków and Gdańsk by train over a week and you are looking at 5 GB. Two weeks in a hire car around the Masurian lakes or along the Baltic coast, with the phone doing the navigation, is a 10 GB trip. The 20 GB pack covers a month, or the first weeks of a university exchange for a student arriving from outside the EU.