For just the falls, I think one day is sufficient. If you also want touristy stuff, there's a good bit of that around that could take up a few days without being too boring, assuming you're into that sort of thing.
New York has some fine state parks as well. Letchworth state park in particular comes to mind as being not super distant from Niagara Falls (maybe something like 150 or 200 miles) and being spectacular. If it were my trip, I would spend one day at Niagara Falls and maybe two or three at Letchworth, though preferences do vary.
Crossing into Canada isn't too big of a deal if you make sure you have all the required paperwork in order and don't attempt to take prohibited items across the border. You do need passports for the people and rabies vaccination certificates signed by a vet (not just collar tags) for pets. Gun regulations are quite different in Canada; handguns are generally not permitted, and there are regulations and paperwork for other firearms. Alcohol is generally not a problem (in reasonable quantities.) It would be well to look up all the regulations for specifics, of course; this is just a very brief overview.
The border crossing can sometimes be quick and sometimes be slow, depending on the traffic, one's luck, etc.