People frequently ask me how long a trailer should be. I typically say it should be a long as it needs to be unless an external party imposes a limit because of finite time in something like a press conference. But despite this, I still aim for 90 seconds for games, and 150 seconds when I make Hollywood style trailers for games for fun.
When we watch trailers online, we can clearly see its duration either before we click the thumbnail, or immediately after it starts playing; we see the numbers slowly ticking down on the bottom of the screen. The only time we don't know a trailer's duration is when we see them on television, in movie theaters, and during live events like press conferences, livestreams, and award shows.
I think knowledge of a trailer's duration affects the audience's desire to start watching, and whether or not to continue watching after they start.
Trailers and videos to advertise games and movies generally fall into one of these ranges, and here's how I think of them in terms of food:
0-30 seconds - Snack
30-90 seconds - Appetizer
90-180 seconds -Entree
180+ seconds - Feast
I think this food analogy works because think about how easy it is to consume entire playlists of very short videos. Before you know it, you might've spent more time watching Vines than you would one trailer. If I'm at a party, I'll end up eating a lot of chips or snacks without even thinking about it, hor d'oeuvres I won't eat as recklessly as I do chips, entrees I have to be selective about, and a feast is a real commitment.
The shorter a trailer is, the more likely someone is to feel like they have "room" to put it on their plate and watch it. This is why my eyes always drift down to the duration and/or progress bar of a video; I want to know if I have room on my plate during a typical day where so many things fight for my attention.
In addition to duration, I think a video's title affects the audience's expectations before they click to watch a video.
Think of how the inclusion of these words affect expectations:
In the case of 3 through 6, I think a longer duration is preferred because the audience for those videos are looking for a deep look into something they're already invested in. Also, the pace of those sorts of videos demand more time whether for in-depth explanations of game mechanics, interviews or narration, which means a 5-10 minute video makes perfect sense. Imagine if an E3 gameplay demonstration was only 2 minutes long; that's not enough time to let the footage breathe, and for the audience to digest what you do in the game. By the same token, how thorough can a behind-the-scenes interview be if it's only 90 seconds long?
Contrast this with the prospect of watching a 4+ minute long trailer for something you've never heard of. Whenever I see trailers with very long durations, I think: "Ugggghhhhhh, this video is probably going to be a slog." This is because I know the word "trailer" implies a certain style and pace of editing which is very hard to sustain past 3 minutes. Behind-the-scenes videos take their time to talk in depth about one or two things, but trailers are typically a lot of short clips, this means the longer the trailer, the easier it is to show or spoil vast amounts of a game or movie.
This is why I think a good length for a trailer is around 90 seconds for games or 150 seconds for movies, because it's what we're used to, and any shorter or long draws attention simply by falling outside of the norm. Unless the trailer is by Hideo Kojima, in which case it's the norm for the trailer to be 5-10 minutes long.
Both the duration and label of the video calibrates the audience's expectations, which will make them more or less receptive to it depending on your execution. Don't prime them for an in-depth look and give them something superficial, and don't prime them for a quick montage, and give them a long drawn out explanation.
One more thing to consider is: a lot of this is thrown out the window if the audience is already invested in the game or movie they're going to watch a video for. My food analogies go out the window if it's something I'm very excited to know more about. There are limits to how automatically I'll watch any video about something I'm looking forward to, but it gives a lot of leeway.
All this to say: make your game trailers 90 seconds or fewer :P