It won't be months.  It may be a week or two.  Like all manufacturers, their first priority is complete guns, then complete uppers or lowers, then individual parts.  Right now, demand is still very high, so parts orders are running a bit behind.

-Troy

A few people are waiting months for uppers, because they want some configuration that requires a low-volume barrel that Bushmaster doesn't have in stock and doesn't have time to make.  For example, a fluted 24" barrel, or an M4 Dissipator, will take a while, until they have enough extra capacity to run those barrels.  Anything with a 14.5", 16", or 20" HBAR will ship almost immediately.  So will some of the more common lightweight profiles.

I had to wait over a year for a pre-ban fluted Dissiaptor barrel during the Y2K crunch.  Fortunately, it wasn't my only upper...  

-Troy