Your chiller will have a fan and this fan will draw air into it to remove heat from the aluminium fins. The aluminium fins are tightly pack to increase the surface area. Dust is easily trapped or stucked onto the fins as the air is not so clean. After a long while the efficiency of the fins will be not so good and will take a longer time to chill the water. So need to do chemical cleaning on the fins. This is easy to DIY.
Also, once a while, need to flush your inlet/outlet of the chiller. Normally I use vinegar. Just leave about 10-15 minutes and flush again with running water. Lots of minute coco worms will comeout...
My pacific cool chiler has been running for 4-5 years without ever topping up the gas. I just do the aboev only every 2 years.
If you have filters, clean it every 2 months. Every a year or 1.5 years, you can buy the cleaning acid at the refrigeration shop to clean the cooling coils. If it takes longer to cool down your tank, may be need to top-up the gas.