How to know when it's time to do the maintenance?
When you see the temperature suddenly drop on you which is mean your block has a build up. If you use proper coolant it will last you over a year.
Ok, if you load temp is 65°C with HK 3.0 vs 61°C with CPU-370 (mine) then it's about right since you have higher ambient temp.
However, your temps are still not that good if you compare to Kuhler 620 and your 4.5Ghz, 1.280v vs 5Ghz, 1.50v, thought.
Is there anyway you can measure the delta temp of your coolant?
You should have at least from 5-10°C in Delta if your system working proper.
What is your radiator fans speed?
GTX360 loves high fan speeds, so crank it up.