If you can leave the props on, and torqued properly, I would think they will last for years. Mine usually fail due to impact with an immoveable object, long before they fail due to age or anything DJI has done, or not done.
Make regular inspections checking for cracks, chips, frayed edges. As long as they are clean and clear of damage your good to go until you spot a problem.
Do not store in direct sunlight, in over heated or really cold area for extended times as these will effect the durability and flexibility of the molecules in the plastic. That can shorten their life cycle.