How long does it last??

Impossible to answer.
FWIW, the one in my rig is 43 years old and works fine.

I'm on the other end of the spectrum.... mine only lasted 4 years and shorted out, due to aluminum scraps getting in during the construction process.

I see you have two threads up about your converter. I'm going to assume you have some 12V problems.

Here's what I would do to diagnose the problem:
Disconnect shore power.
Disconnect wires and/or clamp from the negative post of the trailer's battery.
Put meter leads from positive to negative terminals on battery and measure voltage. This will give you a reasonable estimation of the health of the battery/s.
Replace negative wires and/or clamp back onto battery.
Connect shore power.
Put meter leads back onto battery terminals and measure voltage. This should tell you if the converter is supplying power to the battery. A voltage of minimum 13.5V should be present.

If the above shows that the converter is not supplying power, look for a circuit breaker on the battery->trailer wiring. Turn circuit breaker on load center that supplies 120V to converter off, then on. If you still don't have 13.5 V to the battery, you may have blown an internal fuse inside the converter or on your 12V distribution block. These will be larger fuses (40Amp) and may be marked "reverse protection".

Hope this helps.

Random. PowerMax has a three year warranty.
My WFCO 8955 did not work right from day one.

My guess is they should last for about 15 years, unless subject to overload, short circuits, someone putting in the battery backwards, applying 240 volts to the 120 volt RV or a host of other reasons that would not be covered under warranty, including lightning strikes nearby.

Fred.

The Elixir that came in my Aliner lasted less than two years. That is the only converter I have ever had to replace after owning many RVs.

I have one 20 years old and still works like it did new.

I have had one that was over 15 years old worked perfectly. I have also had one that went out in less than 3 year. I guess the question is how lucky are you?