This question is addressed to those directory owners that started their directories as free ones and then turned them to paid only. When exactly did you take this decision? Was it when the number of visitors reached a certain limit, or when you gained a bigger pagerank or when?
After having 200 links in my directory ! I decided to turn into paid directory to minimize the time time spent on approving free links
About a month after running it for free and only after I installed a new custom template for it. Spam was getting to me.
I don't have problems with spam, the only thing that bothers me is that I waste a good deal of time with the submissions. On the other hand, if I turn to paid now, I might not get much visitors or submissions due to the fact that Google didn't upgrade its PR and I'm still a 0.
Never. We still got a free option after being in business for almost 2 years. It helps to get some content into the directory.