While online video is not going to replace television anytime soon, it is now decidedly mainstream. About 150 million Internet users in the United States watch about 14.5 billion videos a month, according to the measurement firm comScore, or an average of 97 videos per viewer. Although the Web lacks a standard for video measurement, comScore says average video durations have risen slowly but surely in the past year, to an average of 3.4 minutes in March.