Right from the start.
If you have ads on your site from the beginning, your readers aren’t going to complain when you add them in. (Though my advice to the whiners would be to read the article in their RSS aggregator.)
Unfortunately, when you’re starting out, there aren’t a lot of options for ads. You have the ubiquitous AdSense, then you have affiliate programs, and a sprinkling of smaller ad networks.
My advice: Use AdSense in the beginning, and see how well it works with your site. Use some affiliate programs lightly as well (for products you recommend, not just because it’s an affiliate program). Eventually, when your site gets to a reasonable level, transition from AdSense to direct-selling ads (e.g. 125×125 ads).