At DC frequency, how long can wire be until signal attenuates almost completely?
22 awg wire has a resistance of about 16.14 ohms per 1000 meters and has a max amperage of 0.92A according to this source.

Lets say you have a purely resistive load and maximize your current with 0 wire length, your load would be 9V/.92A=9.7826 Ohms. Using this as a voltage divider you would have:

250 ft - &gt; 2.7 V drop
1000 ft -&gt; 5.6 V drop
5000 ft -&gt; 8.0 V drop


This are very rough numbers, but it is what I would use as an estimate.

Low AC frequency usually acts a lot like DC.

Please note though that you probably have a ground wire that has to go the same distance, so treat my lengths as "round trip".

I don't know if my method is 100% correct, but this is what I do.