ago this was but I'm sure it has been a couple of years. (Car has 240748 miles as of this AM.)
My Boxster's coolant level has been very stable.
From the time I bought the car until the car lost a radiator from me hitting some road debris and all the coolant was lost I never added any fluid to the cooling system.
After the radiator was replaced and the system refilled with fresh coolant the level remained good for years. I had to replace the water pump but not because of any leak, but due to the noise the pump made, the play at the water pump pulley/shaft, and a tiny hint of anti-freeze residue on the pump's casting.
(BTW, Porsche techs tell me that signs of anti-freeze residue alone is not proof the water pump is bad. They tell me Porsche allows for a tiny bit of leakage from the water pump but there's a callout on this. IIRC no fluid, no moisture is allowed to be seen just a tiny bit of residue and the location and size of this residue stain is specified. If the stain is too big or located in the wrong place, the water pump is bad.)
Anyhow, after the water pump was replaced the coolant level remained ok until the cap went bad. Afterwards, the level remained good right up until the coolant tank split.
Since then -- I can't remember when this happened but my WAG is last summer maybe -- the level has been unchanged and right where the tech had it when I picked up the car.
In fact years ago I bought a jug of Porsche anti-freeze and keep distilled water on hand and I don't think I've even used any anti-freeze from the jug and the distilled water I used (about a gallon?) to bring the level up when the cap was leaking. The rest of the distilled water has gone into my cars' windshield washer reserviors.
So, I don't buy into these cooling systems losing coolant over time, unless perhaps the water pump seepage that is I'm told allowed by Porsche is at the very high end of what's allowed and this accounts for the drop of the coolant level over time. Just a WAG understand. Until I get some hard data on what's allowed or tolerable I'm going to assume any fluid drop is a leak somewhere and needs to be addressed.
Sincerely,
MarcW.