I could be wrong here, but the way the code is written, the circuit length should only matter if...
- The AFCI protection is provided by a receptacle type device installed at the first outlet.
- A supplemental arc protection circuit breaker is installed at the origin of the circuit, and a receptacle type AFCI device is installed at the first outlet.
210.12(A)(1) allows the circuit to be protected by a "combination-type arc-fault circuit interrupter", without any mention of the length of the circuit. So I would expect as long as you're installing a combination-type AFCI, and not a "supplemental arc protection circuit breaker". Then there should be no problem. But I've been known to misinterpret code from time to time.
National Electrical Code 2017
Chapter 2 Wiring and Protection
Article 210 Branch Circuits
210.12 Arc-fault Circuit-Interrupter Protection.
(A) Dwelling Units. All 120-volt, single-phase, 15- and 20-ampere branch circuits supplying outlets or devices installed in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, or similar rooms or areas shall be protected by any of the means described in 210.12(A)(1) through (6):
(1) A listed combination-type arc-fault circuit interrupter, installed to provide protection of the entire branch circuit.
(3) A listed supplemental arc protection circuit breaker installed at the origin of the branch circuit in combination with a listed outlet branch-circuit type arc-fault circuit interrupter installed at the first outlet box on the branch circuit where all of the following conditions are met:
b. The maximum length of the branch-circuit wiring from the branch-circuit overcurrent device to the first outlet shall not exceed 15.2 m (50 ft) for 14 AWG conductors or 21.3 m (70 ft) for 12 AWG conductors.