Power conditioner -- when to replace?

Sadly MOV are not easy to check if they are any good. Best bet is to just replace them. They are dirt cheap. and a whole pile are usually stuck in the spot. Just replace them and even add a few more.There ARE other kinds of surge protection. You would need to ask the maker what sort your unit has (if any)If you do not know what they look like, very distinctive blue flat disc. Google "images" MOV and you can see hundreds of them.

Push two wires together .

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? 

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

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

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

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

I would ask you to consider another option.  Rather than "filtering" the incoming power to your system, how about regenerating a new sinewave with THD from the power company virtually eliminated?  Since PS Audio recently came out with a new line of regenerators there should be some P5s or P10s out in the pre-owned market.  I run my entire system, save for my powered subwoofers, off of one P5 Power Plant and am only using 34% of its capability.  The incoming sine wave from the power company has 1.7% THD.  After regeneration, there is barely .1% THD.  Purer, cleaner music with natural dynamic swing.  Full Stop.

I'd talk to Furman about this.