Whether its sitting still and keeping it on for 20 minutes is the challenge or simply looking at it from a financial perspective, many people want to re-use a sheet mask. We do recommend against this, as the sheet mask has been absorbing dirt and bacteria from the skin, therefore if you reapply at a later date, you could be placing this back onto the skin.
The other reason why we would recommend against this is down to the dryness. The reason we recommend a sheet mask is applied for 20 minutes is because that is the perfect amount of time for the ingredients to hydrate the skin, however if a mask is left on too long (such as if you were to fall asleep with it on) then the mask can dry out and it would actually have the reverse effect, absorbing moisture from the skin. For this reason, it could be assumed that applying an old sheet mask may have a negative effect as it will have dried out and may not be able to help your skin.
One alternative option you could consider, if you feel there is too much serum on your packet, would be to squeeze out the excess at the start before you apply it to your face, into an air tight container. This way you can use the serum at a later date without the fear of dirt or bacteria being caught in the mix.
If you feel there is a fair amount of excess serum but don’t want to save it for a later date, you could also spread it into other parts of your body, such as your neck, hands or chest.