Under ideal conditions, about three and a half miles.
For a point source against a dark background, and with a fully dark-adapted human eye, the threshold for human visibility T (in footcandles) can be found by this equation:
ln T = .0828 [K ln(B)]^2 + .194[K ln(b)] - 9.73
... where B is the background brightness in nanoLamberts, and K = .4343. A perfectly dark, clear night sky will have a brightness of about 100 nanoLamberts, and this equation is valid only for backgrounds of 0.1 nanoLamberts or greater, which is essentially totally dark.
Solving this equation for T, the threshold visibility against a background of 0.1 nL gives us 5.3 x 10^-5 footcandles, which is the brightness of 1 candle at 18,792 feet, or 3.6 miles.
Allowing for atmospheric attenuation will reduce that number a bit.