it's a waste of time to attempt to "rehydrate" an old mat. I soaked mine and it was no different afterwards, just wet.
For the hydration to work it has to be before the mat dries out. It's more of an up keep not a fix. To wait for it to be dried out would be like watering a plant dead from no watering. Hydration will help a self healing mat to heal longer but does nothing for hard surface mats. Mats are a consumable product. To think a mat will be good as new for years is not realistic. I give my old mats to DH to use in his shop. All his works surfaces are covered in mats.