Keeping them multiples longer than their intended life isn't wise.
If something is filtering chemicals and metals it will saturate at a certain point, so all you are doing is keeping a chemical saturated object dangling in your water feed. Eventually (concentrated) bits of it are going to start shedding back into the water, somewhat defeating the purpose of ever having it.
Might as well buy bottled water that has already been filtered to a certain consistent standard.
How will concentrated bits of it start "shedding" back into the water you are drinking when there's still a filter in the way?
All that happens is that the flow rate from top to bottom reduces. There are no sudden deposits of metals and chemicals that miraculously jump into your drinking water.