Tire life on OE tires varies according to how you brake and accelerate. Wickers get around 5-6,000 miles on an OE rear tire, gentle riders get approaching twice that much.
No one has ever reported the OE tire as getting "unsafe" until the cords started showing, so you don't need to sweat that aspect.
Nobody ever said they were particularly great tires either.
But please feel safe to wear them out thoroughly, just go ahead and pick your replacement rear tire and have it on hand for "when the time comes".