Re: Sailing from New York City to Montreal ... how long?
I'd have no interest in heading up the Richelieu, so unless you need to get the boat to Montreal, maybe think twice.
However, as Lake Champlain is where I keep my boat, I can't say enough about how great it would be to sail the length of it. Tons of secluded anchorages and islands to explore, very scenic, good marinas for resupplying. I'd actually go in September if I had my choice when the water is still warm, the winds are generally good, and the vast majority of people are gone.
Lake Champlain is roughly 120 miles in length, so it can be done relatively quickly, but at least a week would be my preference if I was trying to enjoy my time and explore a bit.