Reader schwit1 writes: If you're an advertiser who wants to market a product to millennials, you're going to have to make it quick. A new study by comScore revealed online ads targeted toward millennials have to be around 5 to 6 seconds to be effective, a sharp contrast from the traditional 30-second commercial seen on TV. "The length of time of an episode or a viewing period is really important and has got to be short, otherwise you just won't keep the attention of millennials," comScore CEO Gian Fulgoni told CNBC's "Squawk Alley." The format of advertising may have to be radically changed to reach millennials, he suggested. "You're going to have to make your case literally in a matter of seconds and make sure you grab somebody's attention, Fulgoni said.