Percentage of portfolio sold is secondary. The most important thing is overall return on investment. As an extreme illustration, if over the course of a year you invest $10,000 on 1000 domains and you only sell one domain, that means you'd only sold .1% of your portfolio for that year. However, if that one sale is for $1 million, I doubt you'd be complaining about the low turnover rate.
As an example of the other extreme, let's say you invest $10,000 on 1000 domains as follows: 99 domains at $10/each, and 1 domain at $9,010. Now you sell all the $10 domains at $11 each, but you can't find a buyer for the $9k domain. That would mean that you'd sold 99% of your portfolio at a profit but still lost money overall.
Obviously, these are not real world examples, but my point is that the more important questions are about how much money is being returned on every dollar spent and how much money is being made overall.