Yes, it does immediately start coming out of the water, but it doesn’t vanish immediately. Depending on the surface area, agitation, temperature, etc., the hydrogen gas can stay in the water for a few hours or longer before it dissipates. This is much like carbonated water or soda that contains carbon dioxide gas (CO2). Because it does leave, it is best to drink the water before it goes “flat.”
In an open cup or glass, hydrogen will last 3-5 hours.
At most, hydrogen will last in water up to 10 days if the container is filled to the top – with no air space at the top – and the container is tightly sealed. After the first drink and the container is opened, the hydrogen will last another 3-5 hours.