No one has answered you so. I don't realy know about swans, but I do know that Muscovies and Swans have the same incubation time of 35-37 days. I have Muscovies and they will sometimes go 39 days when set by a Hen, (this was in the winter). It is sometimes hard to set the date of when a hen sets the eggs.
I would say that if the eggs haven't hatched by day 45 they aren't going to. If the eggs have turned dark and are real smelling they are no good. Don;t even open them.
Try candling the eggs. I had to do that with my egyptians. They are a newly put together pair and she sat a clutch. I candled after 3 weeks of her setting tight and sadly all were clear. So I pulled them in hopes of her laying another clutch after accepting the male.