Everybody periods differently so it’s tough to be exact. Our average time is about 8 hours on a heavier flow day. Though some customers need to change after 3 to 4 hours, and others can go up to 14 hours (lucky them!). As your period gets lighter, one pair can take you through the day and night (just like regular underwear). But again, everybody Periods differently, so you need to take one cycle to figure out exactly how often you need to change. Start with a conservative estimate on your heavy days, and check in on your Period underwear after 4 hours. If you still feel good, give it another hour. Keep going until you figure out your limit. Here at Period, we find we need between 3 to 4 pairs on the first two heavy days. And if you leak, please forgive us! Just this once. (Remember how many tampons you have forgiven). Treat the ‘leak’ as a gift and note the hour of your leakage. That is the best indicator of your ‘flow threshold’ how long the underwear will last for you.