Depends on how many loops, how many water blocks, how big your case is, whether you have an internal or external mounted radiator, where the pump is located, etc. A simple system might get by with 4 feet. Something more involved could require 8 feet. You don't want the tubes to be too long, but too short and then you have to use tighter bends which could kink. Since tubing is relatively cheap, buy more than you'll need so you have extra, like 6 feet or so if you are cooling more than just the CPU.