Liquid Cooling Question

When using a liquid cooling system do you keep the same coolant always in the system or do you drain it and put in new coolant every (so much usage time)?

Generally speaking, it's a sealed system and as long as you've got some sort of biocide, there generally isn't a need to drain/refill the system (there are exceptions, of course, such as dyes which break down).

On the other hand, most people who watercool are enthusiasts and end up draining/refilling the loop every year or so just because of hardware changes though...