Mazda have several recommended service schedules in the North American market.
Schedule 1, "Flexible" (using the oil monitoring option) has oil changes at 12 months or 10k miles (16k km).
Schedule 1, fixed has oil changes at 6 months or 7.5k miles.
Schedule 2 is the "severe driving" schedule with 5 month or 4k mile intervals.
Playthru1, oil can deteriorate in the crank case even when you don't drive it. Combustion produces water and a host of other things that can get past piston rings or leak through the PCV valve. Even a small amount can eventually eliminate the oil's ability to fully protect the engine over a long enough period of time. If for no other reason than the sake of your warranty, I recommend you change your oil at whichever interval requirement comes first.
That said, if you want to be really crazy about it, you can send samples of your oil to a place like Blackstone Labs to have it analyzed to see how it's doing. Mazda probably won't honor reports from them if you have to get your engine replaced under warranty but you'll still have that peace of mind.