A lemon will last for 2 days at room temperature but is best kept in the refrigerator, where it will be good for up to 3 weeks. Lemon zest - the grated or scraped skin, used to add a burst of flavor to foods - is best used immediately, before the intense volatile oils dissipate. If you've zested a lemon, squeeze it and use the juice within 2 days - or freeze the juice for later use. (Squeeze cut lemons into an ice cube tray, freeze, then transfer cubes to a bag or airtight container.)