I have not hatched any fully Muscovy eggs (just mules) but I did notice that it takes ducks a lot longer to hatch than chickens. And I don't mean the whole incubation period, I mean from the time they start peeping and pecking to the time they break out of the egg is a much longer duration. It can be a little nerve wracking because it does take so long but as the reply before me stated, it's best not too mess with them. Good luck!
Keep in mind that the 35 days is just a rule of thumb. Some eggs may hatch a couple days earlier and some a couple days later. There are a lot of different factors that will affect incubation time, heat, humidity, egg size etc. Are you sure it's a full blood Muscovy too? Muscovy crosses will hatch several days earlier than full blood muscovies...don't fret, broodies are generally much better hatchers than us clumsy people...
On another note, you only have one egg under her, right? Do you plan on allowing the hen to raise it? If she has hatched chicken eggs before, she may see this duckling as something foreign and reject it. You'll know if she has accepted it when you hear her clucking softly to it. If she does reject it, have a brooder prepared just in case. And keep in mind, ducklings form extremely strong bonds with whatever gives them company for the first couple days. If you only have one, and you are taking care of it, it will forever bond to you and think it is a person (my first duckling is like this and trust me, it is not ideal http://www.backyardchickens.com/a/raising-a-human-imprinted-duckling-howards-story). Your best bet is to get it a friend to keep company, although that might be hard to come by this time of year. You might want to see if the person who gave you the egg will have any other ducks close in age (less than a week apart, they grow incredibly fast). Fingers crossed that the hen accepts it though, then it will just think it is a chicken. Just want you to be prepared if she doesn't...
Howard is a handful! I love him though, he greets me everyday when I come home from work when all of the other ducks ignore me. They are snobs lol
Well if it's her first, she should be none the wiser! Should be funny too, that duck will be bigger than her in a month!
Sounds likely then. When it hatches, look at the feet. My crosses either had orange feet (pekin mom), brown feet (khaki mom), or marbled brown and yellow/orange. Also, the beaks usually would match the moms as well.