How much revenue do you invest in marketing?

I have invested 0$ in my website so far, and it has almost 3000 members and 10 million hits a month. I built up trafic through regular affiliations and top sites. It has worked for us. But if you don't rely on a community to bring you trafic, then investing 20% maximum is what I would suggest. I will probably begin to invest around that monthly in advertising to expand the horizons more.