The length at which on branch circuits the voltage drop exceeds 3% per 210.19(A)(1)(FPN4) or 3% on only the feeder per 215.2(A)(4)(FPN2) or a total of 5% from the OCD of the feeder to the last outlet on the branch-circuit. This is not an actual requirement, but considered for efficent operation. The short way, start doing calculations when lengths start exceeding 100 feet.
This rule of thumb Generally works, but lets look at a common 20 amp 120 volt branch circuit. You can only run about 55 feet at 16 amps before you have to upsize to #10. At about 90 feet you would have to run #8.
This has bit more that one electrician in the butt, myself included.