I don't have any solid rule on that decision. Each one is a judgement call. Factors in the decision include: Don't overspend, upgrading or repairing a poorly performing system beyond what is sensible. But, don't abandon a system solely on the basis of age, if repairs would enable it to perform well and meet your needs.
At what point do you stop replacing parts and just replace the system all together? Or do you never give up? What's your take?    
At what point do you stop replacing parts and just replace the system all 
together?