A compressor is one thing. A smokette is another. A good sized compressor draws 3-4 times the wattage as a smokette. The lower the draw, the smaller the wire or the longer the length of wire you can use.
According to my official wiring code manuel, you should be able to run 90 feet on 14 guage wire at 5 amps (or 600 watts) power draw with 2% voltage drop (apparently a percentage deemed reasonable without negative consequences). Now the 500 watts that the smokette draws is only 83 percent of the amount from the table, so theoretically you should be able to have a longer cord or will experience less voltage drop with 500 watts.
What also comes into play here is the distance from the main panel to the plug-in in terms of actual wire length. That must also be considered in the voltage drop. But being that almost all outlets are wired with 12 guage wire, the voltage drop to the outlet should be minimal. One other thing that must be considered is any other draw that is ahead of the smokette on the same circuit. This will produce a voltage drop and the total wattage draw on the circuit must be considered then.
Hope this helps.
Mn Que