You may have noticed but there isn’t a timer for QuickTime recordings and if you are trying to meet a minimum (ie. for YouTube), you may find out that sometimes, you come up short so here are some ways to keep track.
If you have multiple monitors, use one of your free Internet browsers and navigate to Google.com. There, you can use their timer (to limit) or stopwatch (for tracking) with ease.
If you do not want to use the Internet, Siri can help. Ask “her” to set a timer for you in which she will inform you that she can’t and instead notify (with a popup) you at the time your timer would end.
Your Android or iOS device has apps that will help. Keep in mind that if audio is recording as well, it may pick up audio from your device.