How often should windshield wipers be replaced?

I always hear a noise when scraping, and then I change it.

If it doesn't clean properly, just replace it.

Replace the weatherstripping separately.

I usually buy two pairs for 39.9 and replace them casually every few months.

If it doesn't clean properly, just replace it. Poor quality ones last about three months, while good quality ones can last over a year.

I haven't changed it for 3 years.

There is no set replacement cycle for wiper blades. Generally, if the blades are worn out and leave streaks, it's time to replace them, and they're not expensive. If it's just leaving a film on the glass, you might want to consider removing the oil film first.

If it doesn't clean well, just replace it.