This quote is suspect because it doesn't sound like Franklin. It's a bit too short and direct for the old master.
And the word "lunch" didn't come into common use until the 1920's. The word was always "luncheon" until then. Franklin, in his era, wold have used the word "supper".
It does appear as if the source is undocumented but it is a worthy observation!
Here is some interesting information related to searches for the source:
http://answers.google.com/answers/threadview?id=389308
As to the length of the quote, here are some very short Franklinisms in Bartleby:
http://www.bartleby.com/100/245.html
An interesting aside! (Do you work for Colonial Williamsburg?!! I once asked one of their "People of the Past" who was impersonating George Wythe and looked like him too, to describe his typical day. Mr. Wythe began his answer by trying to understand my question and we settled on that I was asking him to describe his regular day.)
So I looked in the OED and it does appear that lunch as a synonym for luncheon was not current during Franklin's time. But it's first use was recorded in 1829 and many 19th century usage examples are given.
ML/NJ
In most of the early years in America and in the years of our Agrigarian society, supper was the evening meal, and dinner was the mid-day meal.
Lots of farmers in Wisconsin and Minnesota still use those terms. We grew up with that.