[Mobhack]

See the list Don provided, then decide for yourself.

Also take into consideration whatever night fighting gear your troops (dont) have, typical early infantry night sights are about 5 hexes. Later ones may be 19, and some 21st century riflemen get 15-20. 20 is a kilometre, and was about the planning range for typical (Tank sized) vehicle-mounter infra-red searchlights (typically, those would be 800-1000m, some smaller ones (on light AFV) maybe as low as 300-400m).

When fighting without any night gear and with visibility 1 or so, its difficult for defenders to give covering fires since they can only see 1. You need as a defender then to line up shoulder to shoulder.

1-2 would be the darkest of nights with no moon, 3-4 a typical moonlit night. Otherwise, why have riflemen with 5 vision?!. The table given as a suggestion that someone else thought up thinks a clear night with full moon is 20 odds hexes (a kilometre!). Personally I dont see that at all. It's far beyond the capabilities of even 1960s IR searchlights - so why would they bother with them if you can see 20+ hexes in clear night (according to the compiler of the list).

So to summarise:
1 - deepest darkest night (maybe with rain, or heavy clouds)
2 - otherwise moonless night, or moonlit with rain etc.
3-4 - fully nighttime with some moonlight.
6-10ish extremely clear night or dawn or dusk perhaps.

The first useful US infantry weapon with an IR sight was the M1 carbine with same, a big clunky thing with a max of 100 yards - 2 hexes in other words. So 2 hexes at night is worth having, and it would only be useful at my recommended deepest night (1 hex vis).