I put a battery in the MG this year. The cheapo Wal-Mart Everstart group 26 battery was on it when I bought in 2015, and had a sticker indicating '08. It came out, and another Everstart went in its place.
It started getting marginal over the winter(when I don't drive the car a ton) and I had to jump it once or twice after draining it trying to get the car started. It started really acting up this spring(the cranking speed was low enough that it would often take ~10 seconds with the car cold, but the battery often wouldn't last that long, although it would hot start the car okay). The fresh one has it cranking with authority. I figure 11 years isn't too bad of a run, though.
I got over 10 years from the factory battery in my Lincoln LS. My MKZ is on its second battery-the current is ~5 years old.
One thing I AM sold on that both the MG and my LS had are remote mount batteries-the MG is behind the passenger seat, and the LS was in the trunk. They're not TOTALLY immune from heat, but they do stay cooler than under the hood. I also appreciated on the Lincoln that the external vent hose kept the battery absolutely spotless. When I pulled the factory battery off, I put a big, hard to miss note on it about ti being bad since it was clean enough that an employee not paying attention could have easily mistaken it for fresh stock. I'll also say that I did nothing to it in that time other than just looking every few months to make sure nothing was amiss. The MG is a bit different since the battery box is open to the road from the bottom.