The amount of data in standby largely depends on your provider.
s1p does not send any keepalive packets at the moment except for REGISTER packets in intervals you can set.
There is no option to see data counters yet but feel free to open an issue should you think this feature to be important.
In Sailfish settings there is already a Data counters app which may help you to determine your network usage. Just measure e.g. for an hour without s1p and the same duration with s1p and you should be able to subtract out the difference .