I don't know off the top of my head but I keep Urliks' guide handy, looked to me to be about 1.5% or ~1 in 75 runs.
Back when I ran countess I would usually do a few hundred runs a night so I would have expected to run into several no rune drops per night. 1 in 75 isn't that bad of odds so even over 50 - 100 runs it wouldn't be that extrodinary to see 3 no rune drops.
sequana said:I've got 3 no drops runs from 280 runs (on players 1or 2)