How long does a water filter last?

We are full-time and on average change ours about every 3 months. We are usually on city water, but you will find that even city water varies as some areas have a lot of lime in the water. We have had a couple of times after using well water (at a forestry park) that there was a funny smell to the water and our shower and toilet flow was low. Checked the filter and there was "slime" in it. We got the bleach out and used it to purify our water system, the filter housing, the inside of our white water hose, the tank and all water lines. So - if you notice a smell or low pressure, check the filter.