how long will it take for my flareon+arcnine egg mix hatches and will it be a arcanine or a eevee
Pokemon Eggs don't mix. Your egg will hatch into Eevee or Growlith, whichever parent was the female.
21 (minimum 5,355 steps) for hatching Growlith.
36 (minimum 9,180 steps) for hatching Eevee.
Every Pokemon that hatches from an egg is the same as its mother, in other words if your Flareon is a female then an Eevee will hatch.
Eevee eggs hatch when you take 9180 steps.
If your Arcanine was a girl then a Growlithe will hatch and that will take 5355 steps.