A Berlin weekend, a trade fair or a month in Germany
A long weekend in Berlin, or three days in Munich with an afternoon at Neuschwanstein, fits in the 3 GB pack, which lasts 15 days. A week on the train between Berlin, Hamburg and Cologne, or a slow drive down the Romantic Road, is a 5 GB trip: timetables, tickets and the map take most of it.
Business travel is where Germany is different. A few days at a trade fair in Frankfurt, Düsseldorf or Hanover often end with the laptop working off the phone's hotspot between halls, and that is what the 10 GB pack is for. The 20 GB pack suits a month in the country, or a family sharing one connection on a two-week road trip. Every size from 5 GB up runs for 30 days from the first connection, so you choose by data, not by dates.