How often you should wash them: After every wear.
Washing guidelines: Since they serve the same function as socks, hand wash nylons regularly. (Hang to dry.) That said, if you’re stuck one day without a clean pair (be honest—haven’t we all been there?), you can pop yesterday’s tights in the dryer for 10 minutes with a Refresh’n Dryer Towel ($10, simplygoodstuff.com), a reusable cloth that deodorizes.
What else you should know: Stay on top of your inventory (and avoid recycling dirty hose) with this snagproof filing system: Place each pair into its own zippered plastic bag along with the packaging sleeve so you can ID the brand, the color, and the size quickly.