It is like a car, fill it once and check the level if you have problems. Normal you check the level once a year. You will not need more than the original bottle you start with.
One of my water cooled computers has a Corsair H70 cooler which is sealed. You can never check the coolant level. The other system I have would have to lose half the coolant before the CPU would start to run hot. So once a year or whenever you have a problem.
if my CPU is running at 50-60 degrees C when I have any browser open + my games what size should i get? C40/50/60/70/100?