Also in the UK I replace mine when they leave significant streaks on a clean, wax-free windscreen and the problem isn't fixed by cleaning the blades with neat screenwash on a rag. At the same time as wiping the blades I clean the bits of windscreen that the wipers don't reeach, from where much can run onto the rest.

This is on a car and a van a few years old, doing no more than about 6000 miles per year between them, and not washed very often. This means they can easily last a couple of years; the limit on the van at least is that the rubber goes hard from being left out in the sun.

Also in the UK I replace mine when they leave significant streaks on a clean, wax-free windscreen and the problem isn't fixed by cleaning the blades with neat screenwash on a rag. At the same time as wiping the blades I clean the bits of windscreen that the wipers don't reach, from where muck can run onto the rest.

This is on a car and a van a few years old, doing no more than about 6000 miles per year between them, and not washed very often. This means they can easily last a couple of years; the limit on the van at least is that the rubber goes hard from being left out in the sun.