At what time would you start thinking about earning money with ads?

Hi everyone,

I am wondering if I should already think about earning money?
One of my German websites attracts estimated 1.000 visitors daily.
More than 90% of my visitors are interested in my field (travel), at least that's what my analysis of Google searchterms shows. And the site is growing from day to day, I add about 5 theme relevant pages per day.

But still, 1.000 visitors doesn't sound like the big money :-)
What would you say, might be a good threshold to start?

Thank you for your opinion,
Eddie