Hi gray2887,
Thanks for using Apple Support Communities! I see you're having an issue playing videos past 16 minutes in length with QuickTime and I'd like to help.
Have you restarted your Mac since noticing this issue? If not, go ahead and do that, as a restart can resolve unusual issues.
Can you play any of these videos with a different app to test the behavior? This article shows how to see if there is another app capable of playing a file: If an audio or video file doesn't play on Mac Look in the section titled "How to search for an app that works with your file".
Did you record these videos or get them from another source? If from another source, contact the source to let them know what's happening.
If you recorded these videos, try starting in safe mode to see if that makes a difference in the play length.
How to use safe mode on your Mac
It may take longer to finish the start up, as safe mode does some system checks, and can sometimes resolve issues. You'll also see some differences in the display. This is expected with safe mode, and when you restart normally, they'll be gone. Test and then restart and test again.
Thanks!