Turn on ads when many people have become loyal to your product. And besides that... the most important part is that put up ads in a way that people don't get bugged by it. Do is slowly and subtly. I used to hate hi5 because they had too many ads and that made the website slow too(on my P3 800 Mhz 196mb ram). It seemed as if hi5 is made of beggars [:P]
And yeah... try experimenting various positions on page to put ads... make colour scheme camouflage with content. do same for the font face size and colour. sometimes put them in the same boxes as content sections... the user sometimes get fooled into thinking that its a content/menu
