My Toyota CHR is coming up for 2 years old and considering how much we rely on the key fob for using our car, how often or when should we replace the key fob battery. 
Currently not experiencing any problems with it but then it could suddenly find returning to the car and not even being able to get into it. 

The CR2032 in the keyless entry fob of our Isuzu Mu-X lasted a couple of years.

 

It gave plenty of warning when it was getting low "electronic key, low battery" displayed on the dash.

 

Read the manual, there should be a way to get into the car (mechanical key) and start and drive even when the fob battery is completely dead.

 

 

I replace mine at 4 years, they usually last longer but....????

You can replace it yourself, easy, Mr.DIY outlets have near any suitable replacement battery.

 

Just keep a spare Cr2025 or whatever in the car.  Ours (for another make of vehicle) only last about 2 years.  If they do 'die' we can extract a 'mechanical' key from the fob and use that instead until we get a new battery.

Most Toyota key fobs use a CR 2032 3V battery.

in UK you can get a pack of 8 in IKEA for £1.00 so I would change the battery regularly at this price. 

I don't know what battery is in the key fob of my Navara, but the car has just turned 10 years and has 200K on the clock, but key fob battery has never been changed

1 hour ago, Susco said:

I don't know what battery is in the key fob of my Navara, but the car has just turned 10 years and has 200K on the clock, but key fob battery has never been changed

Quite possible, bit like a computer, it has a coin battery in it, think a CR 2032 or 25 that lasts for years, just luck of the draw with keys. If I were you I would change it, be embarrassing to be stuck out in the sticks over a 20 baht battery eh...????