It seems to proc very often for me. Also I was mistaken -- it does show up in combat logs. The base damage seems to be 30, and it is affected by mob resistances. If the mobs have high poison resist it won't be that effective. On gray mobs it hits for 29-30 all the time, and averages about 18-22 damage for me on equal level (currently 25) mobs with no known innate poison resist.
I don't know the exact rate of how much dex affects the proc rate, but since my main is a warrior I try to stack str/dex/stam as much as I can on all my gear slots. Even if dex doesn't affect the proc rate that much, it's still worth it to dex stack for the hit/crit rate as landing more hits = generating more threat and readiness. I'll have to try some naked auto attack testing on high health gray mobs like chevron'd bandits or something.
10 pieces of Burlap Cloth worth much more than quest reward....and no next quest available. I will prefer using 10 Burlap Cloth to level up your tailor skills, just delete the quest if you picked it up.