I just replaced the batteries on my boat at the very end of last season. I purchased the boat in October of 2021 and am unsure of the age of the batteries, especially since the one didn't have a date sticker on it. The second was punched with a "3" on it for the year, so I'm not sure if that was 2013 or what exactly that meant, but it wasn't the month. I don't imagine it lasted 10 years, but it did very quickly go from working great to not being able to start over the period of just a day or two. I had an Optima red top in my 1965 mustang for over 15 years and kept chugging along until I let it sit for too long. With my daily driver, I typically see 7-8 years out of a battery before they need to be replaced. Depending on where you live, the kind of batteries you have and how long your boat sits during the "off-season", I can definitely see having to replace your batteries every 5 years or so. When I did replace my batteries last fall, I called up the local GW dealership here in Edmonds and they recommended against going with AGM batteries. They said that they're typically quite a bit more money and with the vibration and bouncing around in the boat, that they don't last as long as the standard batteries. Which I guess makes me circle back around to my DD, in that if it's only getting 7-8 years while being driven daily... I again imagine 5-6 out of a marine battery is probably right around that point of needing to be replaced.