I buy them when they run out. Like all hair ties, they will stretch and break over time. That being said I get a high mileage from them and keep buying more as they run out. I've found that the thick, metal-free elastics stay in my hair and don't start slipping out, which is an issue with thin elastics. The metal naturally catches the hair and pulls it out, since they don't have metal they don't pull my hair unless I accidentally make a few loops and pull the hair out causing tangles.
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