Although they are fastidious groomers, chinchillas don't bathe in water. Instead, they bathe in dust, rolling in it until they feel sufficiently clean. Provide them with dust for a bath every day, for 15 minutes to an hour at a time. Place 1 inch of the dust in a flat dish large enough for the chinchilla to roll in. Don't leave the dust bath in the cage because the animal will begin to use it as a litter box.
For more information, please read the article Chinchilla Care.