I use a 100ft outdoor heavy duty extension cord for my 685watt veg room, works fine. Any cord thats rated "heavy duty" should be rated to at least 15 amps which is all a 120 volt circuit can provide. Ergo, if your circuit breaker doesn't blow, your extension cord is fine.
Hey smile? I'm not calling you out but honestly curious, why would length matter? I would think the entire cord would be charged to a maximum of 15 amps no matter how long it got. Of course if were talking something ludicrous say 600 feet I might see your point? Just curious if ya got some evidence for that, I'd never heard of it so I got thirsty