Tougher people are worth more. As a general best estimate, people at 36 tend to be worth 1 unless they've level sat a while, but can be worth none. Especially newbs that die frequently and kill rarely if ever. Various things guarantee no charges. Eg killing the same dude twice in a row.

Most charges I've ever seen was lilyth (there's a log somewhere). Generally though you're still talking one or two on average even at hero, and less if you are going for low hanging fruit.