Usually fans last about three years before they fail completely If you see one failing sooner than that get rid of it immediately as it may cause permanent damage on your PC if left unaddressed . Anytime fans fail prematurely it is usually because something else failed within your PC such as thermal paste had failed or thermal compound had failed or your motherboard had failed . In any case if any component within your PC has failed replace it as soon as possible as it will only lead your system downhill from there . Replacing fans on PC is usually not worth your time unless its fan or heatsink itself is extremely old or dirty/scratched up in which case just replacing one would be fine , But if its not damaged or dirty/scratched up just buy new ones , Also see if replacing fans can solve other problems like freezing issues or freezing issue .