I'm not a licensed electrician, I'm an electronics technician who's done some of his own home wiring. Please confirm my answer before depending on it. Best I can tell, 12ga can carry 20A "safely" most any distance--that is, without dangerous heat buildup. However, there could be issues with voltage drop and power quality. The charts I saw put the voltage drop at 20A about 1V per hundred feet. So at 500 ft, you still have an acceptable 115V. At 1000 ft, 110V, which is starting to seem a little "soft" to me. While 110V is OK, what's going to happen is that there could be some substantial power fluctuations as loads are switched--maybe not an issue if there's only one device on the circuit. Personally, over 500 ft, I'd start thinking 10ga, particularly to support multiple devices (assuming they add up to a max 20A). Which means for most any residential application, 12ga should be fine.

According to the National Electrical Code the current-carrying capacity of any wire is determined by the temperature rating of its insulation. Common ratings for 12 gauge wire are 60C (21 amps), 75C (28 amps), and 85C (30 amps). But wait, that is not the final answer. Again, according to the NEC, overcurrent protection for 12 gauge wire shall not exceed 20 amps. Furthermore, good design practice demands that no circuit be intentionally loaded to more than 80% of its overcurrent protection. In the case of a 20 amp circuit, that means 16 amps. So even though 85 degree C wire can safely carry 30 amps, you can't use a circuit breaker larger than 20 amps and you can't design the system to carry more than 16 amps continuously (over 3 hours). Your original question was about maximum distance. Voltage drop does play a part here, but it is proportional to the actual load you put on the circuit. The heavier the load, the more the voltage will drop. Conversely, with no load on the system there will be no voltage drop. The worst-case scenario would be to start a large motor (such as an air conditioner) at the end of a long wire run. Motor starting current is usually 6X the normal running current. On a hot summer day when you are most likely to need the A/C, the power company is most likely to be experiencing brownout conditions because everybody wants A/C at that time. In that case your supply voltage will likely be low to start with and the voltage drop at the end of a very long wire may just make it impossible to start the compressor. Even if you do get it started, the ongoing low voltage situation will cause the compressor to draw higher amps. The result will be overheating of the compressor and likely early burnout.

I believe the NEC requires adjustments in wire sizes to allow for voltage drops at 200 ft.