Answer: pour a trickle of water or cover the jar with plastic film and close the lid perfectly to increase the time indicated.
The paints have a limited lifespan. On all pots, an expiration date is indicated which will inform you of the time you have left to use your paint. For pots already started, know that it is contact with the open air which damages the paint, so everything must be done to protect your paint with water from the ambient air. To best keep your paint in water, pour a drizzle of water on top of the paint, without stirring or mixing it, and then close the lid tightly. Alternatively, you can also cover the jar with plastic wrap before closing the lid on it. This will keep your paint for longer. Remember to clean the edge of the pot, especially the grooves, to be sure that the air does not pass and that the pot is indeed airtight.