Duck eggs take 28 days to hatch, so if you don't have any ducklings hatching after maybe 31 days total, they probably aren't going to hatch. At this point I would actually candle all the eggs and dispose of any that aren't developing so you don't end up with an exploded stinky egg mess. If none are developing, then destroy the nest as well and keep her out of that area to break her broodiness. Being broody is hard on ducks/chickens. They tend to lose weight and suffer nutritionally. Some will continue to sit and sit and sit far longer than they should thinking that one day an egg might hatch. It's best to discourage this behavior.
I have often wondered if sperm Was viable for longer inside of a female and it was just “used up” typically within two weeks. I’ve had a similar situation where a chicken came into lay and was laying fertilized eggs 6 weeks after contact with a rooster. Her first 7 or so eggs were fertile.
I wondered myself how long they can hold viable sperm and I didn't think they were fertile until that egg that I totally believe was fertile and her brother was the only drake and she is the only duck left. I thought I had candled all of them but I guess I missed one. There are about 5 left and she is spending alot more time off the best. Today she was off about 3-4hrs and she has been eating well.bathing, swimming, flying a bit and we have been playing together, I am still her Mom,. She has been removing the bad ones daily one at a time and the nest is clean. It is funny how they can hold their poop when they are on their nest???????