It is completely up to you and what you are set up for.
I am located in Southern Maine, so I know the temperatures and how quickly they change in Maine!
My muscovies usually lay until mid-end of september, so if you want ducklings this will be your last chance. Stop collecting the eggs and she will build a nest. Mine didn't start to lay on the eggs until she had a dozen or so eggs collected. If it starts too cool off, don't worry. She will lay on the eggs if she feels the need to. Out of 22 eggs, my hen had 5 bad ones. I'm guessing they were the first ones she laid (she started a nest in April, it was still pretty cool out then). But the remaining 17 eggs all ended up hatching.
You just have to be patient with them. I would suggest getting some hay to put in the area where she is laying. She will use it to build her nest, and it will help keep the eggs warm.
As for the ducklings, september can be really cool (and especially October) for babies. But if you let her raise them, she should be fine. I would suggest setting up a heat lamp in a coop or something, that way she can snuggle them under it if it gets too cold. As long as she doesn't have too many (my hen could barely fit all 17 under her at once), she should be able to keep them all warm.
But again, it's completely up to you. If you don't want to have to worry about them, then I would suggest waiting until spring and letting her hatch some then. If that's the case, keep collecting the eggs everyday. She will still lay in the same spot, but keep in mind that due to the season coming to an end she may stop laying in a month or so, and this is normal for a muscovy.
