I replaced mine about every 18 months, but I did usually get a key not detected or battery low warning a few days beforehand.

Those little button battery’s don’t have much energy in them, I imagine the charge/voltage goes off a cliff towards the end of life.

 

It's really not unusual for a keyless car fob to have a 12-24 month battery life. This is because the keyless function requires the key to regularly transmit (every 10s or so) which the car listens for to determine when to unlock (and the same method thieves use to steal keyless cars, by boosting the keyfob signal so it appears near to the car).

As for why it died suddenly I concur with Tooks having done some work with coin cells - the voltage drops rapidly near the end of their life so the failure point can be hard to predict reliably.

 

I have been warned that there isn't a lot of juice after you get the low battery warning. But your case seems extreme.

Glad you sorted it out! And even better you did not lose synchronization! My spare key was dead, and since then I can't get it to lock/unlock/start.

 

I read about higher battery use with Keyless so just change mine each year (around service time) including spare for what the batteries cost.

 

As stated above, VW’s with the keyless system have a habit of eating through the coin cell batteries about every 9 months or so !.
Last year I went through a spell of changing the fob battery every ten days !.
This happened on two Duracell cell batteries in the same remote !.
I was becoming concerned it was a problem with the car !.
I decided to change to another make to see is this addressed the problem.
Each time I replaced the fob cell, ten days later I would receive the “Low fob battery” warning when starting the car.
I replaced the last battery on the 24th of December 2018.
Still running on this battery !.
It is an “Energizer” CR2032.
Must have had a bad batch of Duracell batteries.
The recommended battery is a CR2025 but with a bit of fiddling, you can get a CR2032 in the there !.

 

I use Energizer CR2025 with no probems

 

There are a LOT of fake "Energizer"/"Panasonic"/"Duracell" batteries out there.
I buy mine from Amazon & have still gotten fake batteries. (Typos on the cardboard printing a good clue!)