(Whoops - looks like someone already responded - but here was my answer anyway)
I'm not a real experienced hatcher, but this has been my experience (always under a momma duck tho', I've never used an incuabtor):
Ducks are due on day 28.  When mine have been under a broody duck they are usually late (2-4 days) and maybe that's because in the first few days the duck is just looking serious about nesting, but isn't actually incubating the eggs (I don't know) 
It seems like sometimes the first pips can come a few days before the ducklings actually "zip" out of the shell, but not always.  Most of my ducklings have hatched - after starting to zip - within 24 hours, but not always.  
If you start to worry about your eggs, make sure that you read the Sticky post at the top of this thread   "Incubating & Hatching Eggs Important Topic Index - Please Review by silkiechicken"  It can be VERY helpful.  Interfering can do more harm than good, but there are those here that do do it successfully and there are links to their posts within that thread.
My most recent problem had to do with the ducklings not being able to get through the white membrane under the shell.  The first one I helped nearly died from bleeders (but it did survive); the second one I helped 
only by wrapping the egg in a very, wet warm wash cloth to moisten the membrane, and then slipping it back under the duck (that duckling was out the next morning); the third one I didn't help at all and it didn't make it out of the shell -- so it's very hard to know what to do.  But those that say to 'LET NATURE TAKE IT'S COURSE!', do have a pretty good point.
Another good reminder is that the ducklings have an egg sack attached that gives them food an energy during the hatching process and it lasts for about three days.  So they shouldn't get too weak after the first day and give up.  So don't worry too much about any quiet 'resting' periods after the process starts.
Good luck --  Hope it's an uneventful hatch and you have lots of babies to enjoy.