How do you distinguish 'funnel leaks' (people who could have been converted by hiring a better sales director) from 'bad leads' (people who would never have been converted because they mistakenly clicked on a deceptive ad).
"Bad leads" are often obviously in the wrong segment. E.g. for B2C either too big, too small, or wrong industry.
Every business and product are different, so there are no specific numbers you should expect in general. You can sometimes find benchmark data from similar companies, but usually you just compare to your own company over time. E.g. if you do a new marketing campaign and conversion rates go down, then you are probably introducing relatively more bad leads rather a sudden poor performance from sales. Although, in the same situation, an influx of a new kind of customer might just need a different sales strategy without changes to product or marketing.