Maintenance I follow based on usage
Weekly: Check for leaks and water levels
Monthly: Dust out the rads with compressed air
6 Monthly: Check the fluid, Run the system for an hour and take a sample of the water checking for bits, dirt build up colour changes etc, I keep a syringe and 2 clean white bowles i put a drop of new fluid in one and system fluid in another
A lot of people also recommend yearly changes, I find this is dependant on usage and the fluid involved. I use pastel and my pc is in a darkened spare room and used on and off some weeks every day others not at all but I do check the fluid out though and would change as and when it looks filthy or theres a change in temperatures