How long does a rain repellent or glass coating last on a windshield?

It varies widely with the product type, preparation, climate, mileage and how often the wipers are used. Wiper friction wears the windshield fastest, so side windows and mirrors usually keep their water behavior far longer than the area in the wiper sweep. Simple wipe-on repellents generally last weeks to months, while dedicated glass coatings are designed for longer durability. Reapply when water no longer beads and clears at speed.