It has been reported many times the average for most domainers is 1% - 3% of your portfolio, with most falling in to the 1% range.
Like @Domain Lead Finder said it depends on domain quality and pricing, but also on how many domains are in your portfolio.
The more domains you have, the better chance you will have one someone is looking for, of course the above statement about quality still plays in to this as well.
You could have a million domains, but if they are crappy domains you will not sell very many.