If the Pokémon holding this item creates hail with the move Hail or the Ability Snow Warning, it lasts eight turns instead of five turns. (Prior to Generation VI, it has no effect on Snow Warning, which creates hail with indefinite length in those games.)
In the Generation VII games, if hail has its duration extended by the Icy Rock, the turn counter on the user interface for these effects will indicate that it lasts for 8 turns for the player whose Pokémon used the move, while it will display that it lasts for 5 turns for their opponent. After 5 turns of hail have passed, the user interface will display that it lasts for 8 turns for both players.