When do you start putting adverts on your website?
A. Steady traffic
When you have a steady flow of traffic coming to your site, you should start considering advertisements.

B. Maintenance is too expensive
If you can no longer afford to keep the website going, advertising is probably the way to go. Some sites are successful with donations alone. Whether this works or not depends on your type of user base. Mozilla, for example, has a very supportive community that actively donates. A site about cars though would not fare so well on donations alone.

C. Interested advertisers
Forgetting affiliate programs and operations like Google Adsense, would an individual be interested in paying for advertisements on your site? If so, you should consider advertising. Other wise you have a lot of work to do before it will do you any good!

Whenever you start advertising, just remember: balance is key! Don’t spatter adverts across your pages - dab them. Flashy banner ads and popups are age-old. Go with non-invasive ads (text only if possible). Remember, content is ALWAYS more important than advertising.