Whenever your coolant doesn't look green anymore.
Yes, if you know what your doing. take off your lower hose coming off of the water pump, and the upper hose and stick the nozzle of your house used to spray water in the upper. run water thoughout the block until it starts to come out clear at the bottom. Then open the bottom petcock of the radiator and rinse that also.
then refill with fresh stuff making sure to get all the bubbles out and doing about a 50/50 mix of water and antifreeze.
Alternatively, Prestone makes a kit you can use.