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And, yeah, I know this is sacrilege in forum89, but so it goes.
If you built the site purely to generate cash, then yeah, wait. But if you built it with useful content, and considered that you might make some money from it - that's slightly different (fine line).
The rule of thumb is, design sites pretending advertising doesn't exist. Build them for the users - make them interesting, with good navigation, sensible, non-spammy links, and people will visit. Then think about AdSense.