How often should I change the liquid in my water cooled PC?
Sure! I'll do my best to assist you with the fluid change interval for your water-cooled PC. 

The best practice is to follow the manufacturers instructions for your specific PC or water cooling system.  However, if the manufacturer did not provide instructions, or instructions are otherwise unavailable, here are some guidelines: 

First and foremost, it will depend on whether your system is a custom open loop, or if it is a closed loop All-in-One (commonly referred to as an AiO). 

The most common service intervals of **custom open loop water cooling systems** are in the 6-12 month range. It may also depend on the fluid already in use, as opaque and/or colored fluids need more service than clear fluids.  Clear premix is always the safest choice.  Clear concentrate or anti-algae plus anti-corrosion additives together with distilled water is also very reliable, provided you mix it correctly.  Be sure to use real distilled water, not demineralized water or any other water, as minerals and other contaminants will cause buildup in your system.  As soon as you add colors you will over time risk discoloration of your tubing, and opaque fluids *will* over time start to gunk up your blocks.  This would require you to open up your blocks in order to properly clean them, since opaque fluids have particles in them that make them opaque, and these particles essentially behave as contaminants.

**Closed loop All-in-Ones**, on the other hand, are usually not intended to be refilled. Most do not even have a port to fill them up. However, they will lose some fluid over time to the air (through the tubing, since any flexible tubing will be semi-permeable, similar to how a party balloon loses air over time, despite being sealed). Eventually, AiOs will loose effectiveness to the point of needing a full replacement.  On the upside, a high quality AiO should theoretically outlast the obsolescence of the PC it is attached to, largely maintenance-free; although in practice this depends on many factors, and some people prefer to keep or upgrade their PC long past its market obsolescence.  Attempting to refill an AiO that does not have a fill port would require technical skills in small plumbing, and is typically not worth the effort and expense compared to simply using an open loop system or a new AiO.  Replacing the fluid in an AiO would also require the same or greater understanding of fluid choice of an open system, as AiO fluids are typically proprietary,  formulated to the system design, and not sold separately as an aftermarket replacement--thus requiring the technical expertise to choose a suitable refill fluid.  

I hope this helps you better understand fluid change intervals for  water-cooled PCs. 

Do you have any questions, or anything else I can assist you with?
Ok, so never for a closed loop water cooler, and every 6-12 months for an open loop, unless using colors and stuff, then sooner.  Got it, thanks.  I have an open loop.  It's pretty brightly colored, but I could just get the same effect with a clear fluid and some colored LEDs.  Oh and it's way over that 6 months by now, maybe over 12.  How do I flush all that out and clean the blocks like you're talking about?