It takes about 5 days to develop a mission (took 6 for the first mission)
It depends on how long the mission is and how hard it is (I do make cheats for my mission to speed it up) Scar-Error's are also a huge problem so of the 5 days I most likely use 2 of the days on fixing errors 1 on actual planning and 2 on making pseudo code (which causes the errors)

So it depends on the map complexety and how "big" it is.
I think the next mission will be done within the next 4 days.

btw.... can u make more??