My vote this week is going in the 26-50% category I think. I will often take 100 shots and come back with about 25-30 that I want to process, sometimes it's more photos and less keepers, sometimes it's the reverse, but overall, I'd say it's somewhere in the low range of the 26-50 category.
As far as how it's changed with time, I think that it's stayed somewhat the same, but my idea of what's a keeper has changed
Depending on the "task" the number of shots may be lower (for example, smoke shots... the keeper ratio isn't nearly as high... more like 1/50). But if I go on the overall view of things, I think that it's slightly increased from when I started because I've actually learned how to plan my shots and execute them the way that I want to (rather than depending on the auto mode
)