I'm no expert, but your standard battery is usually good for 5 years. Even if it exhibits no outward signs of failure, you can have internal problems that let go suddenly, leaving you stranded. Batteries are so cheap that replacing one after 5 years should be viewed as normal maintenance, IMO.
You can get cheap ones at Costco (same quality as any other battery), and they now have the AGM style batteries that are truly maintenance free. They're a bit of overkill if you ask me, plus they cost about twice as much.
PS I just replaced the OEM battery in my 1994 E420, but I thought it had been replaced once before. So do as I say, not as I do