I assumed that you were referring to D&D 3.5, so I answered "a week", though it's not because of
extensive detail, it's because of several other factors:
1. Limited free time. I get every other Saturday off from work, and every Sunday off. I have about two hours of
free time on weekdays (after sleeping, eating, and taking care of necessary stuff 'round the house). This has an impact on me regardless of what system I'm using, though it tends to be more noticeable when using labor-intensive systems such as D&D or HERO. Which is why I haven't been using said systems much lately.
2. Protracted NPC creation in D&D 3.5. There is no 'easy' or 'fast' method of cobbling together a specific NPC for a specific role. The best way to design an important NPC is to use the standard character creation rules which are, regrettably, slow. If you have multiple important NPCs this can be a
severe (and frustrating) drain on free time.
3. Lack of detailed monster creation rules. If the adventure requires a custom-built monster, the complete lack of a true system for creating balanced monsters can be baddening. Those handwavey rules in the MM might as well just say "Make it up and hope for the best!" as that's about what they amount to. When I create a custom monster, I tend to run a few dice roll batteries against it to ensure that it isn't too easy to defeat (or, conversely,
impossible to defeat).
All of that usually adds up to a week (or more) for adventure design in D&D 3.5 -- HERO is
slightly more manageable, because #3 above doesn't apply. The more of Eclipse that I read, the more I think it may eliminate both #2 and #3 above, making D&D 3.5 much more manageable for folks who have to budget their free time (as I do). I'll let you know how that turns out. . . when I get the free time
