Yes, boil as hard as you can until it gets down to what I call "nearup".
Once the sugar content gets high enough you will notice it change. It will foam and try to boil over the pot. This is when you reduce the heat some.
Use a thermometer and see what temp your sap boils at. Once the temp raises 7 degrees higher than what your thermometer reads with pure sap, you have syrup.
It takes a while. The general rule is 40:1. A 20 litre pail should yield you 500ml of syrup.
Any questions just ask.
S.