If you're making a mission with a few objectives and simple dialog on each map, and using already existing maps, it could take 30 to 60 minutes. It also depends on your level of experience with the foundry of course. Keep in mind though, that people want to play fun and original stories. Play some of the spotlight missions and you'll see what I mean.
Personally, I like to create my own environments whenever possible and make them as amazing as I can. Sometimes this means taking a ground map and making it look like a city (as I did in Safeguards) but other times it means taking an existing interior map and adding things to it (as I did in Wormhole Paradox). Honestly, both of those of missions took me a long time to make. Safeguards probably took longer because it has tons of maps and is a very long story. Wormhole Paradox is a bit shorter.
Another example is a mission I just released, the Wizard of Droz, which is a retelling of the Wizard of Oz but fits the Star Trek universe. I really didn't time myself, but I want to say it took me about 3 hours in total. The story was easy because it was based on an existing story, I just needed to create the characters and maps, then put the mission together.