Timelapse video from iPhone way too short

I just spent 3 hours taking a time-lapse video of an important event using my 12 pro max on 16.5 and only got a 35 second video out of it. I was expecting something more like 5-10 minutes. What on earth is going on here? Is it for some reason compressing the video more and more the longer I take a time-lapse? How do I keep my phone from doing this again and not ruining another video like this?