Vehicle respawns when...

[TheSandman]

When a players logs out/is kicked/banned/times out, all vehicles they own should be restored/unspawned.
Not including ones that have been used by someone else with a password, as they might be using it when the owner logs out.

[Joshy]

In my opinion it's better to stop un-needed vehicles spawning in the first place (ahem $nake with his 50+ cars all with passwords) than despawning them.