I said 11%-25% based mostly off my last photo session. As to whether or not I start getting a higher ratio as I photograph longer, I'm going to have to say yes, but only because I find myself taking fewer shots. When I first started, I took well over 200 photos every time I went out, I'd keep about 30, and put up about 5-10. Yesterday, I took 30 photos, and put up 5 (although, to be honest, 4 of them are pretty much the same, just different orientations).
Of course, if you ask this question again in the spring, I may have a different answer for you
