I use cellular almost exclusively as park wifi is generally bad and I don't want to spend by entire morning waiting for updates while at the local McDonald's.
I do "everything" online (including playing poker which does take some added bandwidth due to the periodic ad videos that play) and I average around 10GB/month. I monitor all things individually and our 2 phones and Janet's Kindle only make up about 1GB of that usage so the vast majority if from our computers (mostly mine) during regular surfing. I moved to Firefox and the ad-blocker (now called updated ad-blocker) to cut down on usage as so many sites have so much stuff running the second you click in.