I am on year 4 of my OE VW battery. The heat here kills batteries quicker. But, my engine doesn't strain the battery much. I wish that the EA211 came with a kickstarter or recoil option. I'll replace the batter when I feel an excessively slow crank.
I've never had any use for a battery tester(the battery sales tool).
I've gotten the 'battery needs replacing' at the dealership multiple times when dealing with campaigns, recalls, warranty repairs... and then the battery lasts a year a 2 longer, still didn't fail me, but replaced because of slower than normal cranking speed. My engines are well maintained and start instantly. For a long cranker, one will know soon enough when the battery is done.
For customers, I would tell them that the battery is weak, and will need to be replaced in the future. No one is psychic when and they can plan for it. If they don't want to keep a jumpstarter ready, they can either join AAA or opt for the roadside service option with their car insurance. Many will just ask for the 'replacement'. But, your job as a mechtech or service writer is "highest dollar amount per ticket" so enjoy your sale.