Your video duration all depends on the type of video you are wanting to produce.
When creating a video clip, you first must determine a few things when working out how long your video should be.
Consumption: where will your video live and how will consumers view it (i.e website, Social media, Television)
Content: What is the video show casing? What is your call to action?
Budget: How much budget do you have to spend on the video? The longer the video the more the editing and shooting will cost.
Keeping in mind that consumers have a short attention spans - a short, snappy, creative video with high production values is much more watchable than a long draw out video that looks like your dog made it.
Also, You Tube & search engines value videos much higher that are watched from start to finish rather than partly watched.
A documentary, for example, is way longer than a television commercial because it has a much longer story to tell.
A television commercial is designed to tell a story in a very small amount of time.
With videos such as testimonials, product videos and company profiles, the videos should be short and to the point. A product video can be longer however if you are giving a demo on the product for example a "how to use this product" video. Consumers are willing to invest much more time if they are wanting to buy this product but want to see how it is used or what it does.
There is no hard and fast rule to video production, but the following is a good guide to use: