I'm glad I found this thread. I have 8 duck eggs that were on day 28 today. At day 25 when I put them in lockdown they were all alive. I still see movement in the ones that I've been able to candle through the incubator but none have piped at all. I have no clue when to help or what to watch for.
That decision depends a lot on the conditions of your incubation.
So basic questions: What kind of incubator, how have your temps and humidity been throughout the process, what kind of ducks? How big were the air cells at day 25?