Hard to say, I would need to call my provider, but I can get a rough estimate with my phone. It says that I have used 128 GB this month so far, 75 alone for Crunchyroll, so maybe by the end it should be between 200 and 250. With how much I use my laptop (next to nothing) and Switch (barely any online usage), and taking into account my mother's phone, I should be between 300 and 350 per month, can't imagine anything higher than that.
My bet with The_Liquid_Laser: I think the Switch won't surpass the PS2 as the best selling system of all time. If it does, I'll play a game of a list that The_Liquid_Laser will provide, I will have to play it for 50 hours or complete it, whatever comes first.