As always, the length of your video will depend on its subject and what you're making it for. Instagram Videos can be up to one hour long, but in most cases, unless your viewer is on a long flight, they probably won't look at a single piece of content on their mobile device for that long.
The platform itself recommends that Instagram creators make videos that are short and entertaining. But if your video is 60 seconds or shorter, you could make a Reel. There's no tried and true Instagram Video length, but always trim unnecessary parts wherever you can.