My muscovies and khakis have put themselves to bed for about 5 weeks now. They are currently 14 weeks old. I did nothing special other than herd them into their house each night. Actually, we had to catch them for a few weeks before they started this and it was very difficult to do. They can run really fast and can get in the worst places.
Anyway, the muscovies start heading in about sunset - one at a time wandering in. One the other hand, the khakis line up just before it gets dark and march into their house all kinds of quacking. It's hilarious!