If you mean in Gen V, then I suggest level 45. That gets Growlithe Flare Blitz, Crunch, and Outrage. You can go to the Move Relearner to teach Arcanine Extreme Speed if you want it, as well as Thunder Fang.
If you don't want to wait that long, then any level is fine, as Outrage is a Move Tutor move, and Flamethrower can do for a Fire move (and is probably better if your Growlithe has better Special Attack). It gets Crunch at level 39 if you really want that move, so maybe level 39 instead if you're impatient?