The first thing I should point out is why it has to be based on a length of time at all; why can't it just stop recording a video when the motion has stopped? Well, the reason for that is simply that it is not possible to perform the motion processing on each individual frame coming in from the camera at the same time as video is recording - the two events are mutually exclusive. So we can't detect when motion has stopped; instead we have to record for the length of time that we think we are going to expect, based on the scene you are detecting motion in.
The range of time you can set the Video Recording Length to be is from 5 seconds to 10 minutes. So then why is the maximum only 10 minutes - that isn't as long as I want to record for? Well, firstly there has to be a maximum of some kind, otherwise if it were unlimited there is potential for major problems. Imagine if you had the ability to set the recording time to infinite, i.e. keep going until there is no storage left. Sounds like a good idea on paper, but what if you intended to monitor for a very long time (a couple of days maybe) and in the first few seconds of monitoring someone just walked past the camera for a couple of seconds and then nothing else happened Nothing at all for the next 24 hours. What are you going to get? A video of someone walking across the screen and then, depending on how much storage you had left, a really long, boring video of a completely still and uninteresting scene. Then, when something does actually happen of interest on the second day, you have no storage left to record that all important piece of motion you were trying to detect.
So, the maximum of 10 minutes is there as it feels like a sensible amount of time to expect motion to be occurring or not. In one video, at best you get 10 minutes of constant motion or at worst you only get 9 minutes and 59 seconds of stillness. But the beauty of it is that after 10 minutes, if motion is still occurring in your scene, then it will trigger again and start recording another video, and another after that and so on, and so on. In practice then, the Video Recording Length settings is of little consequence as it will essentially keep recording video for as long as motion is occurring and there is ample storage left - as long as motion is occurring you will be sure to capture it (albeit over multiple video files).
I hope that clears up the confusion over the limit of the Video Recording Length and why it even exists in the first place. It basically protects the amount of storage you are using in the most efficient way possible and ensures you don't miss a thing - you can record as much motion as possible and as little stillness as possible.
