There are many different variables that can effect the distance that you can see with a Night Vision device. The larger the object the easier it is too see. If you're trying to see details we call this recognition range and if you're just trying to see whether something there is or maybe you will just see movement but won't be able to 100% determine who or what it is. This is called detection range. Another variable is lighting conditions.

The more ambient light you have (starlight, moonlight, infrared light) the better and further you will be able to see. You can always see further on a night where the moon and stars are out then if it is cloudy and overcast. We typically state that you can tell the difference between a male and a female or a dog and a deer at about 75 to 100 meters However, if you were looking across an open field and there was a half moon out you could see a barn or a house 500 meters away.