It depends on the Pokemon you are hatching.
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5355 steps.
you can't hatch a pikachu egg on diamond, it hatches in to a pichu.
It takes 9,180 steps or 36 Egg Cycles to hatch an Eevee.
in order to hatch a riolu egg you need to walk about 5,000-6,000 steps
5120, courtesy of serebii.net pokedex (check base egg steps)