I recommend storing the spinach in the fridge in an airtight container for up to a week. The container should be a really good one, with a strong vacuum seal which doesn’t allow any air inside. Also don’t open the container too often, this way the spinach will last for even longer than a week. If needed, store the spinach leaves in multiple separate containers and open only when ready to use. Once the spinach is added to the salad and drizzled with dressing, it should be consumed on the same day. The flavor and texture will change after that and spinach will start to wilt.