In most cases, corrosion is just an eye sore, which can lead to other problems if left to olong. In extreme cases, it has caused plugged up CPU blocks and I heard one case claim it ruined his pump's impeller.
Whenever you take apart your blocks or if it is plexi you will see discoloration or flaking depending on the block. The effect it has is that it will hurt it's performance. When stuff starts to corrode say in a CPU block it prevents the water from flowing through the pins the way it was designed to. This hurts both your flow as well as your heat transfer by reducing the surface area when the spaces between pins are filled. If something is wrong with your loop you can see it in the inside of the tubing in that it will get cloudy and discolored. Depending on what reservoir you have it may also show up there. If you aren't using any anti-algae then you will get the same thing as corrosion build up but instead it will be growth which will result in the same decrease in performance. That warm water is an ideal place for bacteria and algae to grow which is why it is absolutely necessary to have some sort of anti-growth additive whether it be a silver strip or something like PT-Nuke.
Average life span?? Whenever you upgrade, lol. The only part of a loop that is going to fail is the pump which honestly can not be predicted. If you maintain a loop and assuming the pump stays strong it will probably go indefinitely. I can't think of anything that will go bad. The only other thing may be the tubing if it gets really old at the fittings and starts to dry rot. Rubber seals might go bad as well but that would take a while. That might lead to a leak but that's after years and years of wear. The limiting factor and reason most people keep changing their loops is because of a change in hardware or case which means you need a new block (GPUs) or a new mounting bracket if they are available for new CPU sockets.
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