Cookies are best fresh, but a few simple habits will keep them tasting great for days.
Use an airtight container. This is the single biggest factor. Cookies left uncovered dry out within a day, no matter how good they were when they arrived. An airtight container (or a sealed zip bag with the air pressed out) keeps the moisture in.
Room temperature vs. the fridge. At room temperature, in an airtight container, our cookies stay fresh for up to 5 days. If you'd like them to last longer, refrigerate them — properly stored and refrigerated, they're good for up to 30 days.
Bringing them back to life. If a refrigerated cookie has firmed up, a few seconds in the microwave (or a couple of minutes at room temperature) brings back that soft, gooey centre.
Don't mix flavours in one container if you can help it — stronger flavours like Biscoff or Nutella can subtly transfer their aroma to milder cookies stored right next to them over several days.