Do these magnets ever have to be replaced? How long do they last?

You only have to get it once and it lasts almost a life-time.The manufacturer warranty is 50 years. You will get your money's worth after washing ten loads of laundry. (Money saved on buying detergent and electricity)

In the long run, this saves you thousands of dollars over the time to come. It also makes your clothes last longer as you are protecting your clothes from harsh chemicals - damaging to you, environment and fabric.