Well, the adventure is short. It was written as a one shot. There aren't a lot of encounters so good resource management should make rests a non-issue. It's just something that I want you to be aware of up front so you don't blast everything on the first encounter and expect to be able to take a long rest. Also, spellcasters are versatile. If you come up with a clever way of achieving a rest using magic or something else, I'm not going to instantly ban it. I just want you to think that you are in a dragon's lair and once it knows you're in there, you probably won't have much time to hide/rest (again unless you're really clever which is something players excel at).
In terms of time, the adventure is short, but PbP makes things long. I'm guessing with consistent posting the adventure will be done in a month or two. That's my estimate though from no experience running PbP and little experience with high level play. There's going to be a lot of significant combat that slows things down as well. Even so, I still think a month or two should see the group either triumph over the dragon or become his dinner.
