After a few hours the sap will be reduced to 6 liters.
Put the gas lower because if you boil it to hard then the sugars will burn and you will have a bitter burned taste. After 5 hours the juice is reduced to just 1 liter.
Go on until you have only 400 ml over. Filter the sap two times and place the sap, almost syrop, in al smaller pan and simmer it to just 100 ml. As you see it's not a lucrative business but it's a very tasteful.