For good reliable detection of people, motion should be across the sensor, and at a maximum distance of around 20 - 25 feet. For bigger objects, with a good temperature difference between them and the background, you can can get triggers from much further away, but they are not always reliable. Having said that, when you do get long distance triggers, they can be annoying, if they are from say a road at the end of your drive.
IR on max brightness has a similar maximum range, fading into the distance, but on max it will washout faces when they get near, which may make you decide to choose a lower setting. I get good results on medium, but ambient lighting has a big impact on results. Cameras are surprisingly good in low light, so you can get good results by providing white light with a security light, or similar, and setting IR to off. This has the additional benefit of having the recordings in colour. Well, in colour as best as is possible with relatively low light conditions. Independently light the scene very well, and you can get excellent results.
For this to work reliably you have to set IR to off, otherwise, if the camera triggers before your security light, it will be in night vision, and record in black and white, and will not switch to colour when the lights come on. If set to off, the camera may still start first, and you will see little, because it will still be dark, but when the security lights come on the camera will automatically adjust it’s light level, and it will be in colour.
Lots of posts about this if you do a forum search.