It would have to be 3 minutes at most, and that wouldn't guarantee the ShB/EW marks survive longer. Most of these are also spawned by groups of people, so giving one person a buff wouldn't help.

What matters more is when the mark dies. At least over here, people seem perfectly fine when one is pulled after 2 minutes and dies in 30 seconds, but take issue when someone charges in and everyone hesitantly chips away for 5 minutes. You might as well blame the people who are unloading all their cooldowns just because they see the mark attacking someone, as if hitting it once doesn't give full credit.

It being an unwritten rule is precisely why they're ignoring it.