I don't think there is any rule as such, but that's not a bad policy to take. In general it depends on the circumstances though, if at the back of a train leaving the terminus, it's reasonable to assume that once the train leaves the station, unless there are people in the coach looking for seats, then all reserved seats are available. If at the front of a long train, then it's a different matter since someone might have got on at the last minute and be walking through, and oviously if boarding at an intermediate station, time needs to be given incase the person has gone to the toilet or buffet.
Of cource the other thing is if someone comes along to claim the seat. Is it legitimate after a certain time to say that they have "lost" the reservation. Perhaps the main problem here would be if someone reserved from one station but infact boarded at a station further down the line, by which time someone might have taken their seat.
Another problem with reserved seats is when someone comes along to claim a reservation on a different train to what they are on. I've had that once before, when a group of people tried to move me and a few others from "their" seats. Since the seats had reservations that were not from the station they got on at, I asked to see their tickets before moving, to make sure they wern't trying it on, and when they showed me it turned out they were booked on the previous train, so I told them not only was I not moving, but that they would not be advised to make a fuss to the guard since he would almost certainly side with me, and likely require them to upgrade to a walk on fare.
