As soon as you can. I was recently asked for some link space on one of my sites. That tells me its time to do a formal page telling my advertisers the stats of my sites and who my target audience is. List the prices for ads and start collecting.

If you are getting requests for link exchanges you can also use that as a sign to start advertising. If you are swapping links with people that’s good for backlinks, but you can’t beat having cash in hand or PayPal.