I have actually found this game uses very little data. I use it non-stop for my half an hour train ride to and from work, and I'm not even getting close to the data consumption I had when I was religiously on Twitter.
The only real article I could find on this so far was from some weird Canadian site, but the only thing they lie about is their self worth, right?
Data analytics firm P3 tracked more than 200 smartphone owners in the U.S. and Germany between July 6 and July 15, 2016, and found that the average Pokemon Go session lasts around 175 seconds or three minutes, consuming about 300 kilobytes of data.
That means you could play more than 3,000 sessions in a month with a one-gigabyte data plan (provided you did nothing else on your phone, of course).