In the case of mob fates, make them just keep spawning mobs for 15 minutes. In the case of boss fates, make them heal themselves for 20% health each time they fall below 10% until the fate is within 3 minutes of finishing (so boss fates would last 12~-15 minutes, depending.

Odin, Behemoth, and other fates of that level would be exempt as this might make them unkillable lol.

And then, when you look at the map and it says maximum time remaining, it gives you an idea if it's worth running across the map to the silly thing.