I am assuming you are determining the water content of a hydrate by heating a sample to dryness and measuring the mass before heating and after heating. After heating the sample to dryness, you should allow the crucible to cool to room temperature and then immediately measure the mass on a balance. The reason to not let the dried material sit around is because it can re-absorb water from the air upon standing to become a hydrate again. This property of a chemical to absorb water from the air is called hygroscopy. A chemical that readily absorbs water from the air is said to be hygroscopic. If the dried sample re-absorbs water, it will affect the mass and the outcome of the experiment.
If you wait too long, the compound will start to become a hydrate again because over time it'll start to absorb water. That'll skew your data