Here is my definition:

1. Wing(s) completely framed in either 1 piece, 2 halves or 2 ends and a center section.
2. All wing sections covered.
3. Fuselage built up and covered (if applicable) and painted (if applicable).
4. Tail components built, covered (if applicable) and painted (if applicable).
5. Remaining work largely assembly and finish detailing.

Certainly not a binding topic but a fun one. To me, it doesn't make a difference how long it takes but what steps are already done for you that makes it an ARF.