It can really vary a lot on what you're shooting not just how. For example if you're doing a photo booth sit down portrait style event I'd expect to come back with most of the shots being "keeper" quality. Shift to something like wildlife or sports and I'd expect the keeper rate to suddenly drop. That isn't saying that they are less skilled, but that the capacity for errors is greatly increased.
However keeper rate is something that can often not increase much. This isn't because a photographer fails to improve, but actually because they do improve, and that pushes their standard up. Higher standards means that shots which once would have been good or passable are now considered failures.
Heck this can mean that, sometimes, your keeper rate goes backwards.
In the end my view is that keeper rate is something nice to know, but not something I'd really let yourself worry about. So long you are able to shoot an event and feel that you've walked away with enough good shots (or if working manage to meet and exceed your criteria for the shoot) then that is ok in my book. You can push yourself to do better always, but I wouldn't get stuck in the mind-set that one must get a certain "keeper rate".