Power conditioner -- when to replace?

The parts don't age, so much as surges age them, but this aging applies only to disposable units, which incorporate shunt devices like MOV's to short lightning strikes to ground, or neutral.

Furman conditioners which incorporate SMP can last decades, and protect your equipment from aging as quickly. It would take a pretty spectacular set of strikes to age them.

Best,
E

2 years is a lie they last much longer id say 10 or 20 years depending on the unit.

I have a Monster Power on my TV/DVD system that is over 20 years and hasn't been off in years ,

Wow. what a story. "lose there effectiveness??                
My Furman is at least 12 years old. I bought  it used nine years ago.I also bought a used PS Audio P-600 then too.Still in use. still work great.  
Now there IS one small part which DOES stop working. and that is the overvoltage surge protection,(If the power conditioner has this?)  if they are a the MOV type which fail if they get put to use. So generally those would be no good after a few years in general But that does not make the power conditioner no good. just if it used those for over voltage, then they need to be replaced.     
So my guess is the op is confusing surge protection and power conditioning

Thanks for all the input, it is very helpful.  To Elizabeth's last point, the usefulness of a "power conditione" includes both functions, typically.   Frankly, it the surge protection is kaput after a period of time, then one is left with only power conditioner and no protection for their gear in the event of a voltage surge.  I think both functions are equally important.  

Question:  is there a way to discover if the surge protection is still effective in a unit?