the ir-goggles used to be like weapons (in hand), and using them was like shooting, even with targeting and max range.
so, the effect is instantaneous, and reveals aliens that "get hit".
the range is bit more than flamethrower, as far as i remember, probably in the .ufo scripts somewhere.
so, when you use the goggles and turn around, the aliens will be visible only in the direction you were facing previously when you used the goggles. you need to use the goggles again after you turned around to see the aliens in the new direction.
the range is small enough and the scanned area is not clearly identifiable (some goggle applications will probably overlap if you need to be sure), so scanning a somewhat large map with tunnels and things can be quite confusing with "zillions" of applications of ir-goggles before you finally find the stuck aliens.
this is one of the problems i suggest to alleviate by giving more vague data about the sum of distances to aliens that lets you figure out the general 'warmer-colder' direction all over the map.