Lemons continue to respire and ripen after they have been harvested. The faster they respire the more carbon dioxide they produce, and the quicker they spoil. Store your lemons in the fridge to slow this down and they should keep for up to about ten days. Make sure you do not store them in plastic bags where they are on top of each other. When there is limited oxygen, the ethylene gas gets trapped, causing them to rot. Always bring them to room temperature to get the most juice out of them.