We bought our boat in Shediac NB in 2013 and brought it up to Kingston. This was in late Sept/Oct and weather
was rough. We lost
5-6 days to weather. Plenty of gales. It took us 18 days total. Just go straight down the seaway. Use the tide tables and go with the tides. We have a lobster boat and it cruises at 8kts, but going upstream with the tide, we were doing 12kts!
In good weather, you can do the run 10-12 days. There are ports
every 20-40 miles along the south shore. We travelled 10 hour days and would go about 80 miles. Plan a destination
port, and closer port along the way to bail at if the weather turns. The further east you go, the bigger the water
gets. Also tides become a factor for both navigation
and in the harbours. For example, we grounded in Rivière de Loup when entering the harbour, we waited 20 mins and the tide rose and lifted the boat up and we were able to dock
Down of Quebec City there are some tidal rifts (like Rapids) , you want to make sure you hit them at the right time, ie high or lowering tide in you case.
I also highly recommend Cheryl Barr's Down East Loop cruising guide. We used it extensively.