I never know when to replace DH's stuff and worry that I'm being a bit too frugal 
I would replace if they had holes, or the elastic has gone but what if they just look a little faded or bobbly or are well over a year old? He doesn't really care but he does need to look smart for work (not that he ever needs to show his undies to clients but you never know
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How often do you replace stuff?