Your question leaves out an important point: are you looking to get out ASAP, but have an ADSC that will prevent it (unless twice passed over, in which case you're out within seven months); or are you looking to stay in, but that's contingent on getting promoted?
If the former, then the timing of your promotion board is irrelevant. Apply NOW, let your prospective employers decide "how far out" they want to talk to you. Also, look at whatever waivers, Palace Chase, etc. opportunities you may have to curtail that ADSC.
If the latter, you have to ask yourself if you could continue as a captain (assuming continuation is offered). You should also really look hard at Palace Chase--I have no personal experience, but know several people and have heard of others for whom the AFRC provided promotion opportunities after getting passed over on AD. Again, though--apply NOW....