How much used traffic by ADSL modem when not used ?

None..

Is this a trick question ?

You will still have unsolicited incoming traffic that the router's firewall should block and outgoing request for things like email clients if you have them installed, also programs checking for updates. There is no real way to put a number on the amount of traffic. If you think the traffic is heavier than it should be check for malware or viruses, and check the firewall settings.

I was reading "not used" as inoperative, if it had said idle that would be a different matter.

The OP may return and clarify what they mean here.