Well it would depend on the NDB's pwr output, which varies between 25W & 10kW. Obviously the low-powered NDBs related to IAPs, aka locators, won't give anything reliable beyond 20 to 25nm. If you're flying over the sea, U might pick up some high-pwrd NDB signals up to 500nm away, though I wouldn't use them.
Actually, I don't think I've EVER used the ADF in commercial ops... or maybe once in some obscure carribean airfield when the GPS system was having a moment ! Thing is U can't couple the autopilot to the ADF, so it's too much hassle. Enter the coordinates of your NDB in the FMS & forget about the beacon itself.
MF