5 Years, 309 hours and still going on the original battery in my B7510, of course- it may be up to 7 years old seeing as I picked it up as a hold over on the dealers lot.
Last winter it got a little sluggish during our coldest time. I have a HF solar battery tender that works great in my truck, I may pick up a second one for the tractor.
Look at the states of residence of all those who report long battery life, they all live in higher latitudes. Those of us reporting shorter battery lifespans live where it gets VERY hot for long periods of time. Extreme heat makes the acid much more active. Climate is as big a factor as any other in battery longevity.
I don't think OEM batteries exist in Kubota equipment. Dealer installs whatever they want. Could be good, could be cheap. When it's only got a 12 month prorated warranty, it's probably a hint
Both my 2003 Kubota batteries are going strong. With some pretty decent swings in our hot/cold climate these past few years. Our heat wave this summer, I think will be another record, and who can forget the last blizzard?