Minimum length Youtube video to show TrueView Instream ads ?

Hi,

Nowadays I'm into the 7 seconds video world, my channel is doing pretty great with Adsense but I noticed that I could earn more money if there would be some Trueview Instream ads instead of the regular Overlay ads. Is there a minimum requirement for video length for the Trueview ads to be shown ? I never saw any Trueview ads on my 7 seconds videos that's why I'm asking.

kind regards

bump this one. Ok I discovered today that I get trueview ads when the videos are minimum 30 sec long just to let you know.