Istanbul, Cappadocia and the coast: which pack
Four or five days in Istanbul with two nights in Cappadocia, the classic first trip, fit in the 3 GB pack if you lean on hotel wifi; it lasts 15 days. If you keep maps open all day and post the hot-air balloons at sunrise, the 5 GB pack, valid for 30 days, is the safer choice.
A two-week loop that adds Pamukkale, Ephesus and a stretch of the Aegean or Mediterranean coast means long bus or car days with navigation running, and that is a 10 GB trip. The 20 GB pack is for a month in the country or for working remotely from Istanbul. If the phone also feeds a laptop or a travel partner's phone, go one size up. To size it to your own plans, try the data calculator.