I would say “as minimum” ( also see: https://flightsimulator.zendesk.com/hc/en-us/articles/360013463459-Minimum-Recommended-and-Ideal-PC-Specifications-for-Microsoft-Flight-Simulator ) . If you have only 5mbit, just the download of msfs makes no fun 
The overall usage depends on your flight hours and also the regions… there is more or less no limit and some hundred gigabytes/months can be reached ( one example : Data Consumption - is this correct? - #2 by BubbaBlitz7348 ). But it is not allway such high…
Some users report issues with the connection, others (like me) never had an issue.