It varies a lot depending on the length/complexity of the mission. But yes, a month is a fair average.
I wish you guys did weekend speed Foundry challenges. :-)
Most of mine were done primarily over a single weekend. I know some folks take pride on spending months but when I read Goatshark's bit about how Foundry authors get unlimited time, all I could think was, "I like working with artificial time limits."
Last I checked, all were over 4 stars.
Granted, there aren't many weekends I both have free and feel like doing it but I do get a few thousand dilithium a week in tips in a busy week, which ain't bad off 4 missions.
And I had something mechanically novel (at the time) in each one.
My next gimmick I want to play with is using NPC pathing for theatrical staging.
Big thing is I want to try my hand at more solid action missions and fewer novels.
My mission with the retired Vulcan ambassador tried to up the ante on that and mostly succeeded but I need to gut a reading section that drags it down.
I like to think the hard part I've gotten good at is managing to write scripts that reference 15+ episodes subtly and create a mission over a weekend.
And, yes, I know the Foundry tools are not the same as internal tools but I am seriously hungry to get access to things like your internal costume editor. Give me ten minutes and I'd have the start of a Godzilla mission. (It's sad when I have a checklist of things I'd do with your internal tools. Why do I torture myself with this stuff? Why, Taco, why?)