Their poison will often last until you're dead. In the event that you somehow get poisoned, here's a little trick you can use if you're near death. If you're leveling by an Outpost, like the ones in North Gustaberg, West Sarutabaruta, or West Ronfaure for example, run over to an NPC and engage in a conversation with one. Don't complete the conversation, though, keep the dialogue box open, and /shout for Poisona. Make sure to tell your position as well. Or if you're close enough to town, you can do the same trick with a gate guard, and hopefully a White Mage, who will help you, is running around in town.