At first the sap boils rapidly and with pretty big bubbles. It looks like water boiling.
After I have used up all the sap from the big pot on the pellet stove, the sap on the kitchen stove starts to boil down in earnest. All this time I have been skimming off the foam that collects near the sides of the pan. These are minerals, etc. No reason to keep all that foam there.
Next day, same thing and I add the new syrup to the old syrup. If there is a day when I will not get so much syrup out of what had been gathered and to boil it down to syrup stage would have left too little in the pot to control the boiling, I don't boil it down to syrup, but save the partway stuff off in a jar and add that to the next day's batch.
This method of pot watching has worked out for me for quite a few years.