it depends on the event or lack thereof, for me.
An Italian auto show I attended recently resulted in almost all hits, meaning I captured on film pretty much exactly what I envisioned.
A music/art/photography outdoor event I attended last summer resulted in a couple halfway decent shots out of ~48 shots/two rolls.
A Bastille Day street festival last summer yielded a complete roll of personal accomplishments 🙂
A trip to the corner coffee shop or local bar generally yields a shot or two per day/(1/2 roll), but that's the place I go to experiment with hip shots, timer shots, over- & under-exposure, etc.
A once-in-a-lifetime trip to NYC, Europe, etc. yields very few wasted film out of perhaps three rolls/72 frames.
I think alot depends on how I feel about the scene. If it's nothing special really, I am finding the limits of what works and what I can and cannot do. If it is something I am excited about and have limited film, I make every shot count, using all I learned from the coffee shop experiments and bar shots.
There's times I am experimenting and times I am playing for real. I experiment so I can get the shot when it counts. I guess I don't really consider "keeper ratios" in the sense most of you seem to.