Quantity of traffic on test network
I havent seen a proper answer on the internet so I did the test myself using bitcoin-0.18.1 with the -testnet flag and after downloading the chain ~2 GB I monitored the traffic of the bitcoind process with nethogs for 24 hours.

I got 40MB download and 20MB uploads so 60*30 = 1800MB

You can expect 1.8GB of traffic from the bitcoin testnet compared to the 200GB+ they advise for the bitcoin mainnet