Maybe I'm wrong, but I believe there is a bug with Instant Poison (II). I refreshed all poisons one after another (so at most should be 1 min diff), and my charges are still quite high on each weapon, but my time remaining for the poisons drastically alternates (currently 1 is 19 mins, and the other has 2 mins). Same charges though.
I swap weapons frequently, but all have been applied. Time should be separate from charges. Has anyone else noticed this?