It's worth noting that the pvp flag in wow was eminently abusable.
It's what makes me think timers aren't the way to go. Straight up, switching such a flag on or off should only be possible while logged out, and there's no need to add a timer to switching the flag while logged out because they've already escaped any consequences at that point.
I think the context is a bit more important, honestly.
If the purpose of the PVP engagement is to get someone else to cease hostilities, or prevent them from engaging in unwanted activity the other person logging out
accomplishes that goal. Destroying their ship is optional, in this case. What would it matter if you destroyed their ship in a game where credits flow freely? It's only a minor setback regardless, because if a player is engaging in unwanted activity they're going to continue to do so after they purchased a new ship.
If however the point is to
harass another player, it prevents them from being able to do so. Harassment is against the terms of service anyway, PVP or not.
If the timers are made long enough, it mitigates abuse. No matter what, if someone is trying to abuse the system they're going to figure out a way to do it. The point is
deterrence not
prevention.