I have added the Google +1 Button to my website and discovered the new Social Activity section in Google WebMaster's Tools. Apparently, one of the interesting things you can learn about your audience is demographic data. But in GWT, the Social Activity > Audience section for my site (currently 82 +1's), says the following:

Your site doesn’t have enough +1's yet to show characteristics

But I'm not sure how many +1's is enough. Google's official help page for the Audience section offers little insight:

The Audience page displays information about people who have +1'd your pages, including the total number of unique users, their location, and their age and gender.

All information is anonymized; Google doesn't share personal information about people who have +1’d your pages. To protect privacy, Google won't display age, gender, or location data unless a certain minimum number of people have +1'd your content.

But what is that "certain minimum number"? I've tried Googling this but all I could find to date was this page which doesn't answer the question.

So how many +1's does a site need before GWT will show me audience demographic characteristics?