As stated, it depends on the session length, the type of session as well as the photographer's skills, and the photographers pickiness.
For instance, in a controlled environment, like a studio portrait shoot, the level of quality shots would probably be fairly high. The main thing that would keep me from using shots would be that they are near duplicates, or the subject was making a wierd face ( or blinking ).
As for wildlife, you are sort of along for the ride, its more spontaneous and "go with the flow" style, atleast for me anyway. So the number of keepers goes down. Often times your subject is on the move as well which is obviously harder to shoot than a stationary subject.
Personally, I am very very picky and am currently trying to amass a good portfolio for possible publishing, fine-art, and stock photography. I want to have a leg up on a lot of the crap out there nowadays, so I am extremely picky about sharpness, composition, lighting and try to avoid cropping as much as possible. So my level of keepers is not very high. While atleast half of my shots may be "usable" there are probably only a select few that not only meet my quality standards, but are also of significant impact to warrant adding to my portfolio. Also, if I were selling framed fine-art prints at several thousand dollars a pop, I would probably be even more picky than I am now. You have the ability to be even more picky when the payout is worth the added effort and time of shooting with an extremely low yield.