I literally use them till they no longer serve their intended purpose. When I start to feel water leaking in, it's time to throw them away. That could be three weeks, or several months -- depends on how hard I scrub the dishes, I guess -- or how long before I puncture one with a fork!
I also use a trick that I read here at Thrifty Fun -- I'm left handed, so when the left glove wears out, I throw it away and turn the right glove inside out for my left hand. I can save about half on each pair by using that little trick!
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