It boilds down to this: Population over 100 - maybe 1 or 2 ships depending on what else is happening on the server (How many droids, jedis, flashlights, dupes, etc) and no ships ever if 120+.
The only exception is for the arrival & exit of the event jobs, they may arrive on a ship that will be despawned immediately upon landing and the same with exiting. This is because not much at all will be happening and the lag won't be as bad.
As for ships in the middle of the event, it's up to the discretion of the Game Master to take into accounts everything that is happening. The main thing that could be done is do space battles were it's just ships fighting and no boarding - but it has a major drawback. If two ships crash, they have a chance to instantly crash the server and there is no preventing it - so that discourages Game Masters a lot as it is completely out of their control.
Overall, ships need to be looked after the entire time they're spawned and held accountable for what else is happening on the server. So, it is up to the discretion of the Game Master as long as population is below 100. Above that ships can only be used for a few seconds at most when not much else is happening.
This may change when we swap maps or when the lag is decreased further.