Filter pads

How often should I replace filter pads? I run a sump, skimmer and fuge. They seem to get pretty dirty, does this mean they need replaced, or will they still serve the same purpose if they are dirty?
 
if the filters pads are dirty clean them or if they are old replace them (i clean mine several times before replacing them)
they still filter your water as long as the water has to flow through them but if you let the food and waste build up your just helping to add to higher nitrates
 
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