According to this blog post, the average review takes just over 2 minutes. Change the format to replay officials (similar to college football), and I think you can cut that down significantly. But for argument's sake, let's say the amount of time for a review doesn't change. In 2013, the average NFL game lasted 3 hours and 10 minutes and featured 1.6 reviews. Let's say the number of reviews goes up to 10 per game, which is probably a stretch, you're looking at about 18 more minutes. You're now looking at the average NFL game lasting 3 and a half hours. Not a material change, especially when you consider that added time comes with more accurate officiating.
Yeah ... the average time was 3:10 for the actual game plus 50 minutes of complaining about all of the bad calls. Now it will be 3:30 with only 10 or 15 minutes of complaining afterwards, so the overall time out of your day will be shorter. ;)