Well, unfortunately weather conditions play a big part. First batteries don’t like the cold. Secondly you really don’t want battery problems in below freezing temps, even at a Walmart! Depending on where you are, I recommend a new one every 3-4 years. Even if you sell or trade you’ll get a discount for an old battery (at least it will become an element of negotiation. AAA, flashers, jump starters, etc. are good to have but you really don’t want to use them.

 

I think hot is much worse for car batteries. But - as you say weather, wheee you live, duty cycle all impact aging of batteries. So it's hard to do preventative as some may do 5-6+ years and others 2-3. Seems wasteful to change out a battery that might give a few more years
I've always waited for the battery to show signs of deterioration and then make a shift. I find they give good warnings before failing.

 

Has anyone found battery with bigger reserve capacity?

 

These batteries are BIG, HEAVY, and EXPENSIVE. Whether you do it proactively or reactively, it's a pita. If, like me, you wait until it dies - you'll have an exciting time getting into the vehicle and a harder time removing the passenger seat so you can remove the battery. If you do it ahead of time, then you can get it out without removing the battery (just move the seat WAY forward and up before disconnecting the juice). The trick, if you're cheap like me, is predicting when is a good time to replace it without allowing it to die. My dad had a HUGE aversion to having a beef cow die on our farm. It's kinda like that. If you have a wiff of the end coming, take it to the stock market. Me, I now keep that thing on a CTEK battery tender whenever it is the garage. I think that is the best thing you can do for a Defender's battery longevity. Ymmv.

Oh, to answer the original Q: If I hadn't replaced mine already, I definitely would around the 4 year mark.