after 3 dungeons in a row on "silly huntard surv dual wield shit toon" - it's about 5-6% of dmg and according to quick math from Skada dmg meter - about 2 ppm (most weapon proccs are 2ppm now afaik)
edit: ur right ofc, it can procc back to back if lucky. but there is a per wpn icd of 2 per min...