My ratio varies, based on time of year, how many stands I do during a hunt, where I hunt, and if we're counting all predators (foxes, bobcats, badgers, etc).
When I first got started in predator hunting I could've given you an exact figure because I kept a little black book of predator hunting with meticulous notes. Now I'm a little more loose with my record keeping, so I'll have to guess. Roll it all into an average, and I'd say about 1:5 stands I see a predator. Some days I call an animal every other (or damn near every) stand, and some days I go all day without seeing anything after 12 or so stands - hence the average. I don't hit stands where the wind is wrong, the setup is bad, or where it just doesn't 'feel' right. And like ch1966, I've been in slumps before - longest one was probably about 15 stands and zero animals seen.
Which is where I feel a need to make a point - just because you aren't seeing them, doesn't mean you aren't calling them in. Make sure that your setup allows you maximum vantage of your area, whether that's several hundred yards or just several yards. Make sure you can see as much of your downwind as possible and that you have provided security routes for predators to approach from. Get into position as quietly as possible and leave as quietly as possible. Use every terrain feature you can to your benefit, and don't forget to use decoys where they can be seen.
And sometimes, you just can't win. A coyote will bust you, a bobcat will see you, or that fox just won't come within shotgun range. You are hunting predators, and the challenge is what makes it fun and exciting.