The optimal length for a video advertisement can vary depending on the platform, audience, and message. However, in general, it's recommended to keep video ads relatively short, with most experts recommending a duration of 15 to 30 seconds for a good video ad.
This is because attention spans are generally shorter for online video content on social channels, and viewers may quickly lose interest if an ad is too long or doesn't hold their attention. That being said, longer ads (such as 60 seconds or more) may be appropriate for certain campaigns or platforms, such as TV or pre-roll ads on longer-form video content.