Guide
Ask five marketers and you'll get five rules: 15 seconds, 21–34, under a minute. They're all answering the wrong question. The right length isn't a rule — it's a measurement.
TikTok's own guidance favors 21–34 seconds for In-Feed ads, and there's real logic behind it: feeds are impatient, completion rate feeds the algorithm, and most ads simply don't have 60 seconds of material. Short formats force discipline.
But the averages hide the mechanism. Short ads win on average because the average ad has dead segments — and cutting an ad short happens to amputate them. The variable that actually drives performance isn't duration. It's whether attention survives to the end.
Your ad has a natural length: the point where its predicted engagement curve sags and doesn't recover. Everything after that point costs you completion rate, algorithmic favor, and CTA visibility while delivering nothing. Everything before it is earning its runtime.
That point is different for every creative. A demo with three strong reveals can carry 50 seconds; a single-joke UGC clip exhausts itself in 12. Applying a universal length rule to both wastes the first ad's material and pads the second's.
Upload your full cut to PreTestAds and read the curve: Attention Drop marks the second predicted engagement falls meaningfully below peak — your candidate cut point. Purchase Signal tells you whether viewers are still engaged during your CTA window; if not, the CTA needs to move earlier, or the section before it needs to go. Trim, rescore, compare. Two or three iterations usually finds the length where the curve stays strong start to finish.
Start with a strong opening (see how to write a TikTok hook), and validate the whole workflow in TikTok ad testing.
Upload your cut and see exactly where attention drops — first analysis free.
See Your Attention Curve