I think the 40 days may come from one of your privileges, as you may know, not to be called to serve on a jury. Another privilege of members of Parliament is not to answer a subpoena to appear as a witness in court. Those are good for 40 days before a session and 40 days after a session. Apparently, it is a tradition that grew up in England.
Normally the proclamation would be issued at least 40 days—or around that, as it varied at different times—before the session began, because people came quite long distances, and in the 1800s and before that, travel wasn't that easy. Then you gave them time to get home, so you gave 40 days afterwards too.
So that may be what the reference was to, that if you are going to prorogue Parliament, there is no point if you are going to have the people go home and then come back again. You want to give them time to actually get home and then to come back again, so the prorogation would have been longer.
Sessions used to be very short, even in early Canada, because this was not a full-time job then. People had farms and they were doctors and lawyers and had their own practices. They had to travel. Some of them still do. They had to travel a great distance to get home, and there was not that much legislation; there was very little government legislation in the early days of Confederation.