So my newest version has a counter for each use of the poison (every time it coats a weapon), once the counter reaches a threshold it uses up one of the poisons in the barrel (goes from 5 stacks to 4, for example).
Once the poison runs out, it deconstructs the "building" and gives an announcement, returning your barrel and letting you place another poison barrel in the same spot.
I also made it affect all wielded weapons within it's area, friend and foe alike, to add some tactic on where you place them. Don't want an army with helmet snake venom running amok. The venom will also naturally wear off weapons like any normal contaminant, so you'll have to keep a source of poison if you want it to be a mainstay in your military.