Hi and firstly welcome to the Tropical Aquariums Forum
Now down to business lol
I personally run 3 of these new FLUVAL 3 internal filters but can only give you a rough guideline as filter cleaning all depends on size of tank, real or artificial plants, stocking levels (no not the nylon type lol), feeding levels and the kind of fish that you are keeping.
Basically i always clean internal filters every 2nd water change or when you notice a drop in flow rate but as for external filters well i can leave them for upto 4 months at a time before they require cleaning (i dont use activated carbon in any of my filters so i dont have to replace it at 2 week intervals) but obviously if the flow rate drops i clean them sooner.
You will notice on the top of your FLUVAL 3 is a yellow indicator that measures the flow rate this is a very good advancment over the old FLUVAL design as the old type did not have this feature, when the yellow indicator starts to dissapear into the filter housing is the time to clean your filter sponges, you will also notice the flow switch on the front of the filter housing leave this at maximum flow as it is best for filter performance and oxygenation of the water in your tank
Just a foot note............always clean all of your filter sponges etc in the tank water that you have siphoned off whilst doing your water change as if you use tap water you will kill all the helpful bacteria held within the sponges and end up with dead fish after about 2 days of putting the filter back in your tank!!!! also do not wash the sponges too well as you will still wash away upto 75% of the bacteria.......i advise just lightly washing your filter sponges as this will not wash away too much of these helpful bacteria
By the way do you use the oxygenation device that was supplied with your filter and if so what is your opinoin on it please?