Rose that depend
if the rooms are mandatory to complete on time - or only the build is
if the build have (lets say) 5 rooms, predecided by you
I see 3 options I would feel was ok
1) you sign up and have to meet the deadline for every room and then the end of it all the house.
Then I think many would need 2 weeks pr room + to finish the lot = 12 weeks
2) if the rooms is only mandatyr for the build, and don't have to be turned in on time (unless you want bonus changes, if it was a competition) then I guess a week would be ok.
3) (like this best) 2 weeks pr room, and you can choose to do the rooms like now - so you don't commite for the whole thing, But IF you do you end up with a house.
though I might not be the best to ask as I mostly would use this as inspiration (why I like version 3)