I believe it is 5 months, but I'm not currently sitting at my desk for the solid answer.
We had 2 hired for 18x, was advised by NGB that they didn't fill all of their UPT slots (don't ask why, cause I don't know), and were switched to UPT. I believe I heard last week that those 2 will now go 18x due to their packets not getting approved quickly enough to make it in this FY (again on NGB for the backlog). So it looks like it is FY dependent.
BL is you will most likely know when applying and/or when interviewing if the position is UPT or 18x. But, as I stated earlier, expect 18x. Most of the RPA UPT slots I know about are for locations where the OG also had RC-26's.
There may be some opportunities for new hires to go to UPT if they do a stint with an ANG MC-12 unit and then return to their ANG RPA unit for permanent assignment. This is only a proposal, and right now doesn't look likely.