Incubation is 22-23 days, according to The Birder's Handbook. So, it's probably another week or so at least before the next hatching. I stopped by there yesterday afternoon. One parent was incubating, and the other was feeding 3 of the young ones. The fourth juvenile looked a bit bigger than the others, and was off doing his/her own thing.
There are 3 other adult pairs there. One pair has started building a nest on the island just east of the footbridge, next to the beaver lodge.