I refer to the Natasha Trigger in Lake Rathetear.
She should only despawn after a set amount of time, unsure on specific I think 5 min is a safe bet though.  (Whatever the TD version is currently set to stay up for).  Currently on P99 she is despawning after the first person turns in a damaged crown.  Evidence:
September, 2000
	Quote:
	
	
		| (NOTE: If you kill Schmendrick Lavawalker & the spirit of flame first, you will get a damaged crown. More than one cleric can turn in damaged crowns to Natasha Whitewater after she is spawned, but only the person who turns in Lord Bergubble's crown will get a potion Effect Flame Arc - use unknown yet.) |