Without knowing the rpm the engine was turning, there's no way to tell. So, I'll make an assumption, for the purposes of discussion, that the engine was turning 600 rpm. If you watch the video it appears the engine made 4 complete cycles. I figured this out by counting the intakes through the right valve. With this information, the following will tell you how long the video was, in real time, and assuming 600 rpm (note that I chose 600 because it makes the numbers easy).

600 revs per minute divided by 60 seconds gives 10 revs per second

4 revs divided by 10 revs per second gives .4 second.

So, assuming the engine was turning 600 rpm, the entire video was .4 second.

You can thank my 9th grade science teacher for this analysis, he's the one who taught me the method of making such calculations. It's easy once you know the method.