Lots of places will say an average car battery's life is somewhere around 3-5 years. It is highly dependent on weather. Here where there are regularly long spans of 100F+ days, a battery will have done pretty good to make it five years; many die in 3-4. Same in very cold climates. If you're in a place with good weather all the time they'll last considerably longer.

I've heard about this being a known issue with cars from other manufacturers, so I can believe it. It's interesting that nobody thought to include a way to let the vehicle know it should stop trying to communicate to handle a potential end-of-service situation like this. It's fairly common for people to keep cars for more than a decade, they're a really expensive necessity for many.

Maybe I'm used to harsher weather in the Midwest.

That's a known issue. Reports all over the subaru forums.

I tried to measure it myself with my little multimeter and now I don't have a working multimeter.

One suggestion I tried that seemed to work was to not keep the key close to the car, since that'd not physically possible for me I wrap it in an ESD bag. Haven't had much issue since, but no promises.

Depends on the battery and how it was (ab)used. I have seen batteries last only 3 years or as long as 7.