Its tiny. I use it on my iPad when travelling in hotel gyms for running and its a fairly small stream of data as the app is device based and its just exchanging updates for position and power etc. What its really sensitive to is latency rather than bandwidth.
As of a year ago, a network traffic analyzer showed that we used something in the range of 90MB to 200MB total per hour for Zwift. I haven’t tracked that info recently.
Data usage also depends on if you are in an event such as a group ride versus free ride or individual workout. Riding with a group uses much more data than alone. I have used over 4 GB during a ride with a large group versus MB of data usage when riding alone.
You can just free ride by entering the game and riding. If you want the bare minimum of data use, you can enter the game and either start a free ride or workout and then disconnect from the internet. Do your ride then reconnect at the end to upload your data to Zwift. I use to do that when we had no more monthly data and I wanted to Zwift. Of course you miss out on the social aspect of Zwifting. To save data I would also ride along with a group ride and chat with friends in the ride using discord, chat with ride members using ZML, or just ride along. That saved a lot of data and you were Riding along but not in the event module.