MOVs

Please be more specific.... What are MOVs,... video files?

Metal Oxide Varisters

https://en.wikipedia.org/wiki/Varistor

And nothing keeps forever!

Both/either, it is a function of the duration and magnitude of the overvoltages that it is subjected to.

A properly designed SPD (Surge Protection Device) will fail open before it restricts the downstream flow of normal power, but of course your circuit is exposed to the next surge. An SPD that fails shorted will ultimately burn free or trip the upstream protection depending upon how the system is coordinated.

is there a suggested change out interval? or just wait for failure?

Wait for a failure and have the replacement on-hand. I always buy them in pairs and put a sticker on the installed one describing exactly where the replacement is...

Practically speaking they are all or nothing.

They absorb a transient that might otherwise destroy your equipment.

Realistically speaking, I have heard some say that a moderate hit will degrade them, although I haven't seen this.

That being said, because they are so cheap, I would replace one if the protected equipment took a hit.

I was hoping someone would mention valves; I couldn't figure out what it was otherwise.

Exactly, there are too many things out in the world for people to think tossing TLAs⁽¹⁾ out is SOP⁽²⁾. With multiple phrases sharing the same TLA, communication quickly becomes FUBAR⁽³⁾ until we are all SOL⁽⁴⁾.

Notes:

1. Three Letter Acronym
2. Standard Operating Procedure
3. [Fouled] Up Beyond All Recognition⁽⁵⁾
4. [Straight] Outta Luck⁽⁵⁾
5. Edited yo PG-13 version
   

MOV in electrical engineering cannot be VALVES.

They clip extra voltages above normal usually that is and suddenly they are gone.

Always respecting the data sheet parameters/limitations statements; I have found these devices to be as reliable as zener diodes when used properly. MOVs provide some size benefit with respect to zeners for a given power level. These are very robust devices. Generally a vendor will specify the power level that the device will survive continuously and a level that will damage the device.

MOVs are not just used for incoming supply surge protection or power grid lines.

Lots are used in electrical and electronic equipment.

I recommend a web search for "Littelfuse MOV".

On Littelfuse site you will find lots of data and application notes.

They will last a lifetime if run well within ratings. As others have written, on power lines, what hits them could exceed the design levels and cause degradation.

They are not open/short "all or nothing" in the way one thinks of a fuse. Nor do they have a short circuit failure mode like a diode or transistor. They work by "bulk" properties, not junction effects.

They do absorb a little power at normal rated voltage. For small electronic applications, the DC voltage at 1 mA [pulse test at specified temperature] is typically the rated specification. Current will typically increase 1000 times for each 1.2 to 1 voltage ratio step or fall a similar ratio for a voltage step down. At a given voltage, temperature increase raises current, so you know that an overload will not further increase voltage.

67model

We repair many SCR (thyristor) assemblies. Many of these have MOV's across the assembly. and when the explode there is much arking damage. I have only seen MOV's "final fail" be a rupture of the assembly as it it blows apart.

It is quite common for MOV's to degrade over time with on going "use". A "brand new" MOV has a very sharp knee at its breakover voltage. As it absorbs multiple hits this knee becomes rounded, the device then starts to leak more, and eventually the start of the breakdown voltage is at the AC line voltage and it explodes.

If there is ringing with high over voltage spikes the MOV's in these assemblies don't last many years. If there are no spikes at the switching points they tend to last decades. If you are "hit with lightning" they are usually damaged, as in explode (along with lots of other pieces of electronics).

I don't know what the problem is when the OP used the term "MOV"...The relevant engineer folk here would instantly know what the OP intended. The others only rely on Wiki and google to which the OP should have done in the first place. _{(Go ahead...flame me now)}

My direct answer is to check the manufactures data sheets as there are too many variations to give a broad answer. They all behave differently under differing conditions and applications.

You could also try "Transient Suppressors" data sheets also from Littlefuse, Vishay, Infineon, Fairchild.

From a small mind.

http://acronyms.thefreedictionary.com/MOV

Or should I say, for a small mind.

Whatever...

From those 17 listed, 14 can be eliminated instantly in relation to this website. Then considering the context of the OP there are only 2 left.

No need to put others down to make one look/appear higher.

You should read your last sentence .... Try to understand it, and self apply it.

Engineering is a large field, the are a lot of engineers in the mechanical field that look at other fields as a whole.

Whatever? That's a response children say.