I change wiper blades at each service, so 12 months max, perhaps 8k miles and IMHO they are shot. Approx £20 from the dealership.
An acquaintance recently had to renew the blades on his SL Mercedes, £200 the pair! Apparently they are 'active' blades where the screen wash is passed through the blade directly onto the screen
I'll stick with £20 a year, maybe even sooner if I feel the performance has dropped off