There is some conflicting and mis-information in this thread.
First, on the Eheim recommendation that half the media be replaced every 3 months. I don't do this, simply because I have well planted tanks and it is not necessary. If the filter is running in a non-live plant tank, I would suggest this is probably a good idea.
The comment "do not clean the filter media ever" is not correct if by this you mean never rinse it out. All media must be rinsed out regularly, and the time between rinsings depends on the tank, i.e., how much crud is being removed from the water, as this clogs the filter.
Water changes should be weekly. The filtration has nothing to do with water changes. A change of water is a stand-alone maintenance.
Eheim filters function as mechanical filtration, biological filtration and sometimes (depending upon the media added) chemical filtration. I won't say more on the latter as it is the other two that are significant with respect to the "cleaning" aspect in all aquaria, planted or not. And chemical filtration should not be employed continuously in planted aquaria but only as the exception to remove medications or chemicals after disease treatment.
Mechanical filtration requires a steady and clear path of water through the filter. As sediment is removed from the water, it is trapped by the various media, first by the ceramic disks, then the pad, then some by the biological rock material (Eheim calls this EhfiSubstrat, Fluval has BioMax, etc), then the final white pad. All of this media needs regular rinsing to remove the suspended material or the mechanical filtration will clog and slow the filter flow, and sometimes water will find less-resistant paths around it which defeats the whole purpose of the filter.
If the filter is also serving as biological filtration, it is critical to rinse the media regularly. In planted tanks, this is less important because the plants do the main job of filtration and are actually hindered somewhat by excessive biological filtration. In non-planted tanks biological filtration is crucial to a healthy aquarium, so I will comment a bit on this aspect.
First some terms. Nitrifying bacteria are autotrophic, meaning they make their own food. They do this by chemosytnesis, using oxygen and either ammonia or nitrite. Ammonia and nitrite are nitrogenous wastes, which is why we call these bacteria nitrifiers.
There is another group of bacteria we call heterotrophic, because they derive their energy from breaking down organic compounds. These bacteria need to be controlled, because they limit--and can even easily eliminate--the autotrophic (nitrifying) bacteria. And this can easily occur in the filter. Some of the heterotrophic bacteria are de-nitrifying bacteria, so named because they use nitrate and produce nitrogen gas, and these are actually very useful. But the "bad guys" are the ones we are talking about in connection with the filter.
As the water passes through the media, nitrifying bacteria (nitrosomonas and Nitrospira and probably others) colonize the media. As they do, they "pile up" so to speak, and it is possible for those underneath to die from lack of oxygen which can only come from the water continually passing through. [This is one reason for replacing half the media periodically.] As the nitrifying bacteria die, they are replaced by the "bad" guys, the heterotrophic bacteria. And these heterotrophic bacteria can live in high oxygen environments and in low-oxygen environments--so as the water flow reduces through the filter and the oxygen diminishes, not only does this cause the autotrophic nitrifying bacteria trouble, it enables the heterotrophic "bad guys" to increase further still. While we're on the topic of oxygen, the useful de-nitrifying heterotrophic bacteria can live in no-oxygen areas because they are facultative anaerobes, meaning they can function with or without oxygen.
But worse still, the accumulation of sludge--all that brown gunk you see in the media when you clean it--is readily used by more heterotrophic bacteria. Heterotrophic bacteria are principally limited only by the available organic carbon. And this they readily get from all that sludge, so the more there is, the more they reproduce. And, heterotrophic bacteria can multiply much faster than autotrophic, further reducing them.
This situation if left unchecked can significantly reduce the nitrification or biological filtration. Ammonia and nitrite can then quickly spike, something they would not do under optimum conditions in a properly-functioning filter. As I mentioned earlier, having lots of live plants can offset this to some degree, depending upon the fish load, but without live plants this dirty filter situation is very liable to cause serious trouble and quickly.