I a have often heard the question of when is my RO come to the end of its lifespan, The correct answer is when a RO cannot reduce you TDS to 10% of the taps TDS THEN ITS SHOT. EG My Tap water is 310 TDs so my RO when is good working order should produce a TDS of 31 or less.
Now when its reaches this point `chuck it yet it will still work. But understand you di will be working harder and more costlier to produce pure.