Youtubers come to watch short videos, where the info is given quick and the message delivered promptly. Most of the time audiences will judge whether or not to watch the whole video in the first 6 seconds. For comedy (like myself) keep around 1 or 2 minutes. However gaming is different, most people wouldn't watch a 1-2 minute gaming review or walk through or what have you. I see a lot of gaming videos anywhere from 5-20 minutes. it all depends i guess. Use youtube analytics to see how long you retain your audiences attention, and judge from that how long you think your videos should be.