This.The answer to this question is zero percent. The coolant will become conductive eventually through contact with the metals used in the heat exchangers and fittings.
If the coolant being conductive is something you're worried about, liquid cooling probably isn't for you.
Edit: Unless you use something like mineral oil as the coolant. I could see that staying non-conductive for as close as makes no difference to forever. It'd be messy as heck, though.