We recommend you refill the Gentoo daily to ensure the water alkalinity is high and calcium residue doesn’t build up making it difficult to clean.
Water loses it alkalinity in general over 18-24 hours, but this depends on the methods used to alkaline the water, the amount of pollutant particles surrounding the alkaline water and what temperature the water is kept in.  Air carries PM2.5 and PM10 particles in it, so as soon as you unseal an alkaline water bottle, air gets inside the bottle and begins reducing the alkalinity of the water.