This is something I've been thinking for quite some time. As you said, there are absolutely some games that benefit from long videos: games like MGSV or The Witcher 3 that have a lot of systems/depth and need more time to be shown off properly. On smaller games, however, it seems far too common that the crew keep going well past covering what a game is and just want to keep playing because they're having a good time.
This is around what I think. It really depends on the game. Some videos do not need to be past even a half hour, let alone 15/20mins while some games it would benefit to be longer.