I've only done builds myself with clear coolant and they seem to last a good year or so before I even feel the need to drain, flush and re-fill, and I actually sometimes think I could go longer. I'm just helping a friend with a build at the moment though and he wants to use a white Mayhems Pastel coolant which I've not used before... how long would you say he'd be able to go with that before having to re-do it? I've read the Pastel ones can be quite regular. And what are the indicating signs that you NEED to? Higher temps, colour changing?