How do you know when to change the battery in the key fob?

What is your question?

One time a message appeared on the screen that the battery in the key fob was low. Has only happened once several days ago and has not appeared since. I certainly do not want to be caught out somewhere and not be able to start the car. According to the book, if that happens the Toyota emblem on the fob should be held on the pushbutton - but then what? The salesman at the dealership told me to carry both key fobs, but that sounds a little ridiculous. The car is a 2015 Avalon, mostly town driving so it has been started hundreds of times, with no problem.