My experiences with a popper have always been they roast it too quickly and the coffee did not have time to develop properly. Ive had some roast in 2 to 3 minutes which is way too fast.
Even back in the days when I first started roasting with my I-Roast it'd run about 7 to 8 minutes.
The issue with a popper is, if you use too little coffee, it can fly out of the stack, if you use too much, it does not have the fluid bed mobility and can scorch because it's not moving around. Or since its putting so much heat so fast, its scorching. As for other more normal roasting methods, Id say probably 20 minutes to roast is starting to get a bit long and into the toasting / cooking instead of roasting range.
Each setup is a bit different and your voltage, as you have seen yourself with the cord, can make a HUGE difference with just a few volts. If you can get it around say 5 minutes I think you are doing good. You are going to have to decide yourself by looking at your beans, are they scorching, tipping, or getting too dark?
Not entirely sure your setup but I also used to use a soup can, cut both ends out and use it as like a chimney to extend the area the beans can bounce around in, this can help control temperatures sometimes too as you can put more beans in as necessary to control loft w/o them flying all over the place since the longer chimney keeps them in.
Aaron