You must have a lot of stop and go trips for such frequent tire replacement. Furthermore, Michelin's PS4 and AS3+ have mileage warranties, I don't know about Pirellis. I hope you're not changing the tires at the dealership. Go to Americas Tire, have them teach you how to measure the tread depth. Ask them to give you the little tool and show you how to read it. Once tread depth hits 3/32 of an inch, change immediately; at 4/32 of an inch start looking for tire deals; in USA 2/32 of an inch is legally required; in a wet region like yours, I'd change around 3/32 of an inch. Dealership advises change way too often for their own profits.