Your media shouldn't be changed out, but cleaned in "aquarium water" not tap water, the chlorine and other goodies in your tap water can kill the beneficial bacteria growing on your filter media. There is a debate about how often to clean a canister filter, like Eheim. Some say more frequently, others say you can go for a long time. Leaving a canister filter to "perk" (not clean it) can create a nitrate factory.
First look at water flow, put your finger in front of the output spout or watch the strength of the spray; if you find a noticeable slow down, maybe time for a mini-quick clean. By this I mean take the filter out, and check the sponges in particular to see if they have become filled with gunk (again wash in aquarium water you have harvested from a water change, or just take some water from the aquarium to clean the sponges). Then look at the substrate like material to see if it is really dirty, again clean with aquarium water.
A tip I give is to put a sponge filter over your intake tube of the Eheim. This acts as a mechanical filter keeping much of the crud out of the canister. This creates more work for you but less work in the long run. My Eheim flow will slow down; when I took it apart it was clean but my sponge filter (on the intake tube) was filthy. After I cleaned the sponge the flow was good again.
The sponge I use is the replacement sponges for this filter, so I just purchase the sponges, not the entire filter: http://www.animalworldnetwork.com/el...lter-a902.html
I personally will check my canister about every four months and do a quick cleaning. I know the external sponge on the intake tube helps keep the canister "cleaner." But I do need to clean the sponge filter on the intake tube just about once a week; this is easy, slip it off, rinse in a bowl with aquarium water, slide back on - entire time about five minutes.
The actual media should last you for years. If you add Seachem's Purigen as I do ( I highly recommend this - an excellent water polisher and its cheap), you will need to "recharge" it about every 6 months or so, instructions will come with the package on how to do it - basically, its easy to do and you use bleach. You should get about 6 to 8 times of recharging before you replace your Purigen.
Don't purchase the Carbon filter media for your Eheim, instead the Purigen is a better carbon (it isn't carbon), lasts longer and polishes which carbon doesn't do. Carbon only lasts for a short time. The carbon that came with my Eheim was only recommended for starting a new tank and to throw away after 8 - 14 days of use, which I did.