I have had my pc now for almost a year, i have only ever overclocked a handful of times. This being so, how often should i change the liquid cooling ? I have not had any problems with the cooling itself, all temps are still brilliant at between 30-40.
Closed loop - never.
Open loop - depends on several factors and will be specific to your particular system. Can you see how much water is in your reservoir? Are you loosing a lot to evaporation? If it's open loop, and been a year, it's probably due for a maintenance check up.