For a mission of average complexity done to a high standard I would say a week, give or take, assuming two to four hours invested every day of that week. This leaves out planning and writing the script, which is generally what slows me down.
A campaign, though? No longer than a year. That's a should, of course, and I've failed to uphold it myself, but I think it's not far from the maximum a campaign of around retail length and quality really demands, ignoring FRED-unrelated things like VA and mods. It happens slower not because of the workload, I think, so much as the fact we're none of us getting paid to do it.