The drop rate appears to be about 5% on all worms in the cave and there are sufficient quantity of them that you should be getting them at 1 per clear or 1 per 2 clears. But only if you are killing ALL of the worms including the ones segmented off in the back not connected to the territory of the worms up front.
So go through and clear the entire network of caves once or twice and mark your map if you need to so you know where the worms are at and then just make sure you keep those cleared and you will get them at a decent rate (depending on how many you need of course).
Respawn seemed roughly 10 minutes, but I never timed it.