I had the same problem today when I was raiding C3s for disaster teams. In my experience, it takes about ten game days.
Interestingly enough, SF don't seem to have this problem. Yes, there is the lag of getting them paradropped, but I found that to be about half the time with 300 military transport aircraft organized into the proper units.
I was also able to drop the SF sprinkled throughout the country, meaning overall faster painting. Might be an alternative to consider