just wonderin how many litres i need to change all my coolant when i attempt it on the weekend. also where is the plug that i unscrew to empty the radiator?

hmmm im not 100 % sure but i think its 15 litres the cooling system takes. your best bet is to get the 1 litre coolant concentrate and then top it up with distilled water. and if your not sure what coolant you currently have in there its a good idea to flush out the system too. as for the plug i got no idea, i just undid the lower radiator hose

plug i think is drivers side, need a philips screw driver - but that only empties radiator and over flow tank. to empty the bloke i recomend opening the tehrmostat housing and removing thermostate and running the hose in there aswell. the flushing the hose through the over flow bottle should get it all.
i think off the top of my head the entire system holds just under 15L's. so usually one 5L bottle of concentrate and 10L of water (tap or bottled distilled, what ever you use).
also might want to put some of that muck out/radiator cleaner/flush before you flush her, seems to work some and can't really do any harm...

its basically a brand new radiator as i got a new one when i did my engine 2 yrs ago. how often should the fluid be replaced?

depends on what type is used. generally every two years is recomended. but you can find some long life ones that say up to 5 years. but then it also depends on how much the car is driven, cars doing high milage should have it done more often. also depends were you live, e.g. living up north were it's hot your cars cooling system is a lot more important to keep in good condition than here in Tassie were we have beautiful 14 degree summer days!