Well, in Vermont, at least, the "season" lasts for a couple of weeks at most, so it is only the larger producers who would separate the early sap from the late sap; I had a small stand of trees, and just collected everything for the season, and then boiled at the end. While I wasn't able, therefore, to notice any differences between early sap and late sap, I did notice that the best quantity of sap came on days when it was freezing at night, and significantly above freezing during the day.
Generally speaking, you boil until you reach the right viscosity (specific gravity), which is tied to the sugar content. I used to count on boiling 40 gallons of sap to get one gallon of syrup, but this can vary a bit.
Question I've always had that you might be able to answer: how do you know how much sap you can take? Is there some point at which you're doing damage to the tree?
Generally speaking, you boil until you reach the right viscosity (specific gravity), which is tied to the sugar content. I used to count on boiling 40 gallons of sap to get one gallon of syrup, but this can vary a bit.