Excerpt:

When someone searches using Google Instant, ad impressions are counted in these situations:

  * The user begins to type a query on Google and clicks anywhere on the page
  (a search result, an ad, a spell correction, a related search).

  * The user chooses a particular query by clicking the Search button, pressing 
  Enter, or selecting one of the predicted queries.

  * The user stops typing, and the results are displayed for a minimum of three 
  seconds.