5 percent of Instagram ad impressions lead to 'Likes'

"On the subject of advertising, Systrom stressed that Instagram is taking it slow, and working with a handful of partners like Ben & Jerry’s and watch-maker Michael Kors to ensure the ads are pleasing; he boasted that 5 percent of the ad images have received “likes.” He also noted that location and proximity data are integral to the company’s future, but that Instagram is still exploring how to achieve a balance between the appeal of location information with users’ desire for privacy."
Source:  Kevin Systrom of Instagram interviewed by GigaOM, 6th November 2013
Note - I expect that these levels will inevitably fall as the ads get more commonplace...