Tires, batteries, filters & such are all consumables and I never press any of those trying to get a few extra months or miles out of them, because . . . . they're a major pain when they fail unexpectedly and either leave you stranded somewhere, or they have you scrambling trying to find a replacement after hours or on a weekend, so you never get a good deal or you never find the best brand to buy when you need something in a hurry - Your procrastination has created a mini-emergency and the solution to it is seldom the 'right' one because you're in a hurry. I can't remember the last time I had to go buy a battery because my vehicle wouldn't start . . . . but I know I was much, much younger then and I honestly believed that trying to get the very last mile out of anything automotive was saving me money - It seemed like it was at the time, but in actuality, it wasn't. If I had bought the battery 6 months or a year sooner, I'd have ended up with a better replacement and for less money . . . . and I wouldn't have missed or been late for wherever it was I was headed when it wouldn't start . . . . and I wouldn't have had to change it out in the rain or freezing cold
So . . . . can't hazard a guess as to how long an OEM battery might last - Mine was changed out long ago
Don