In short, as far as you can with your eyes. For example, imagine looking at a building on the horizon – you might be able to distinguish its shape, but you won’t be able to see people inside walking by the windows. This is very much the same for thermal imaging cameras. Whilst factors such as object, resolution and environmental conditions all play a role, most objects distinguishable with your eyes will be visible on an infrared camera.
How far can you see using thermal imaging?