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Theoretically forever on a closed loop system with the proper mix of coolant. But nothing is perfect. I know I lose a little water out of my t-line to evaporation because the cap isn't 100% air tight. I've had my wc system in for over 2years and I've had to add less than 1cup of coolant in that time period. My biggest problem is keeping dust out of my radiator
in what kind of setup? If it's a true closed loop, indefinately so long as the needed anti-biological agents are mixed in. If it's a resevoir setup it will last for a while. For a tub setup I have no idea, but I can attest to how easy it is for coolant to get contaminated in large open containers.
When i first started getting into WCing, i had my closed loop going for about 8 months straight with no water loss and no increase in temps.
I've now upgraded and just replaced the tubing and block, but still the same setup and it's been going for 4 months straight so far with the same good results.
As for systems with reservoirs and fill lines, it might need a top up every few months.
I went almost a year on my closed loop system with no growth or discoloration in the main line. Although, I did get a little bit of white crap at the very top of the fill line so I replaced the water when I changed waterblocks(kinda have to ).