Last month I moved and didn't have an ISP at home for the first 3 weeks or so. I set up tethering on my phone and a 5GB data plan. Yesterday I got a notice that I was over my data limit. I haven't done a whole lot of web surfing in the new house, but I have listened to a ton of podcasts using the Podcast app which I have never done before.
So if I just blew through 5 GB of data in 2ish weeks and the only thing different was using the podcasts app...
(I did watch 3 episodes of walking dead on Netflix which obviously contributed to the data use, but a 45 minute episode can't be that big)