How often should you use a new Gillette Fusion Cartridge?

Well first, your icon pic is SUPER creepy. Lol. You should change it when the little blue strip above the blades has faded or isn't a full line. Usually, you feel it getting dull before that tho. Its really a matter of prefereance... I like using a semi dull razor way better than a brand new one cuz the new one always, always cuts up my skin and gives me razor burn.