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Sending and receiving a turn consumes traffic equivalent to the size of a save file. That is, approximately 500 KB to 1 MB is used when receiving each turn, and the same amount when sending it. The total traffic depends on the number of turns per hour. Sometimes it's possible to play a week in an hour, but more often it's much fewer days. If you play quickly and manage to play a week's worth of turns in an hour, you will send and receive about 5 MB.