Bangkok, the north and the islands
Five or six days in Bangkok, or a long weekend there tagged onto another trip, fits the 3 GB pack, which lasts 15 days. The common two-week route, Bangkok, then Chiang Mai, then a week on an island, is a 5 GB trip, and that pack runs for 30 days.
Three weeks or more, or any trip where you post a lot from the beach, is a 10 GB trip. The 20 GB pack is for a month or more, or for working from a laptop in Chiang Mai or on an island through the hotspot. Plenty of people end a Thai trip by crossing into a neighbouring country: these packs stop at the border, so look at the Cambodia, Laos, Vietnam or Malaysia eSIM for that leg. The data calculator works it out for your own route.