When does a Phione Egg hatch?

Question

I'm gonna hatch it to transfer to Pokemon Black

Answer 1

Phione Eggs hatch in 40 Egg cycles.

Generation IV
10455 steps

Generation V/VI
10280 steps

Generation VII
10240 steps

Generation VIII/IX
5120 steps

List of Pokémon by breeding attributes (Bulbapedia)
Decreasing Egg cycles (Bulbapedia)

Answer 2

It takes 10,240 steps to hatch it. Good luck!