What is the data usage of Splatoon 2 online?
According to one article, they used their phone as a hotspot and found that it used about 150Mb of data an hour. (actually 153 to be more precise).
A Redditor reported yesterday (July 23, 2017) a few sources stating about 135-160Mb an hour. It appears that you can expect about this much data to be used an hour.