It is 10 seconds.
Checking the video is square, under 10s, smaller than 2MB and have smaller size of 800x800 pixel
If possible mute the video that has sound
github.com/telegramdesktop/tdesktop/issues/8304#issuecomment-778014437
If you upload a video longer than 10 seconds, then Telegram will ask you for a 10 seconds part:
