Very axiomatic title, just like beer gets warm if you leave it in the sun a while.
What is the normal amount of time you are suppose to wait until changing a canister filter?
To answer your question strictly, most people wait until the pump fails or the housing breaks down. Some find out that they need to upgrade and then they change
their filter. If you have an Eheim this can be several years or decades. However, you should clean it before it stops dead. Just open it and wash the media in a bucket filled with aquarium water. Cleaning canister filters has been found to help with the flow as well as reducing organics in the aquarium.
I see most people here refer to how often one should clean the filter media and have covered the subject pretty well. After you disturbed the substrate , clean your filter. When you see BBA bloom out of nowhere, clean your filter.
Some folks think that the waste accumulating in a canister is bad for water quality; others disagree. Personally, I don't think the waste causes problems, because I think one removes any excess nitrates and phosphates by frequent water changes. After all, the nitrates and phosphates are soluble.
Most of the aquarists, myself included, start with fish and then add plants. Hence the first things we learn about apply to fish aquariums. It can be quite hard to change some of these believes. When you have fish-only tanks you have nightmares about high NO3, high PO4 etc.
High NO3 and high PO4 alone do not cause algae. In aquariums with high light, good CO2 and fast growing plants you will run out of PO4 and NO3, if you rely only on fish food. Once you run out you will have problems with the plants and algae.
Why did fish keepers associate high NO3 with algae ? Why can we add NO3 to the aquarium and not get algae ? Is there an attempt to fool us ?
Think about it, they do not add inorganic NO3, so all the NO3 comes from the breaking down of NO2, which comes from NH4 - which comes from many other molecules- which come from the gunk in your filter or substrate. It is these upstream molecules that induce algae germination. NH4 and even before that are the stuff you want to remove/reduce from your aquarium. This issue becomes a little less critical when you have lots of healthy fast growing plants which are able to take up some of the NH4. Another way can reduce this substance by cleaning the filter, cleaning damaged plants, the substrate etc.