Years ago, my second year of having cows had a young bull to break his shoulder, then went for a long stretch of several years without any issues. Now recently it seems to caught back up with me. The last few years seems like have to pull at least one bull out of two or three out for some reason or another. I have been leaving at least one bull calf intact and having that option in case I need a mid season replacement.
Some years have a couple, some years one and some none. So I would say 25% of active bulls bugger up on me in any given year. And that is only running them for 45 days in summer and again in early winter. Could be a lot higher for guys running 60-90 seasons.
That's like asking about AI conception percentages. Over thousands of head you can expect to see an average. But it's really a summation of every individual cow. Very similar to rolling dice in that we can easily put a percentage on how many times you'll see a certain number but in practice sometimes we can't roll that number in a hundred tries and other times we can do it every other roll.