What's the general rule about when to replace wiper blades? I've typically just bought new ones when they started leaving wet streaks on the windshield.
If the blade is under 2-3 years old, chances are the streaking is just caused by the blade being dirty. (remember all of the stuff the blade wipes off the windshield has to go somewhere, and that somewhere is usually buildup on the blade.)
I clean my blades with degreaser or 3M general purpose adhesive remover (in a pinch carb cleaner will work too) every 3 weeks during the rainy season. This keeps the blades in top condition. With this regime, blades usually last 3-4 years.
I recommend that you try cleaning your wiper blades before you replace them.
Wiper blades are like underwear, the more you clean them, the better they perform.
Another solution which has worked well with me is too keep your windshield extremely clean. By that I mean washing and then using a product like invisible shield (wipe off with newspaper). Having the windshield clean allows me to not have to turn on my wipers at all. The only time I have to use the wipers is at low speed in a parking lot, other than that, no need. The water just beeds right off.
Coating the windshield is something that I have done for years. I use a product called Rejex on not only the windshield, side and rear glass, but on the entire car. Rejex is an essential part of any Hypermilers tool kit. Not only does rain slide right off, but bugs don't stick to the car either.