I can talk from my experience only. Standard domestic flight reservation for JAL starts 2 months in advance. There are restricted fare “Sakitoku” (先得) which the reservation is available prior to two month of the travel date. Currently reservation for using “Sakitoku” fare is available up till flights for March 30, 2019.
With my limited experience, it looked like domestic first class is not available by “Sakitoku” fare, “Sakitoku” fare is only available for economy and Class-J. On Japanese website of JAL (in Japanese) shows first class reservation for “Sakitoku” fare as「予約不可」, as reservation not possible.
It looks like domestic first class reservation on JAL is available only from two months before the flights. On my most resent reservation I made on JAL domestic flight, I made the reservation more than two month in advance on “Sakitoku” fare and was available only in economy and Class-J. I put the reservation on hold for Class-J, did not purchase ticket. I went back two months before the flight and saw first class availability on the flight.
It may be possible to make a reservation on domestic first class in JAL prior to two months before the flight if you make a reservation as part of international ticket, but I never tried that.