thanks for your three kind replies - I was using the Continental trains rule where it used to be at least the reservee had a certain amount of time to claim the seat - at least put a jacket on it or something before being considered a no-show and the seat was up for grabs - 15 or 20 minutes is or at least was the rule in some countries.

but like annhig says as a rule of thumb I guess if it is a no show for a long time sit in it and then if someone comes just move out - assuming that there were no other unreserved seats available.