It's all in the question name.
256 steps. The chance it resets is not guaranteed, but it has a small chance to do so every 256 steps. If it resets, 40% it will be an item, 40% it will be a hidden item and 20% that it will be a Pokemon.
Source
How to change your username How to add a user avatar Why was my post hidden? Question etiquette Flags vs Downvotes Anime FAQ Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.