So if you think about it, it is not a hard number. Several aspects need to be evaluated.

The first question is the battery getting the proper charging voltage. If it is and the cables are not getting warm then you are probably ok. The variable that defines the voltage at the battery is primarily where the voltage regulator sensing wire is. If close to the battery then the battery voltage will be fine. But if you are pulling off the alternator and have a high drop then you might be feeding your stuff with a higher voltage than you may want.

If the regulator is in the alternator and you can't sense at the battery, this gets more tricky. Now you need much more robust wiring. I would say absolutely no more than 0.5 volt drop with closer to .2 to .3 volts, but that is just my opinion. Other opinions may vary.

Considerations are is the battery getting proper charging voltage, is the stuff being fed from the battery and alternator at a good voltage and are the cables getting warm.

My 2 cents..........