im 35 now cmb 86 but im ranging them. how many hours will it take till 55?

It depends on your luck with getting slayer tasks. If you get the good ones or not. So.. it'll take a while.

As long as it takes for you to get good assignments and kill the monsters.

To tell you the truth, I was gaining levels every day from 60 slayer to 68 (Almost everyday anyway)

I've heard rumours it's a good idea to avoid the metal dragons. But this isn't what brave adventurers like me would do. It doesn't take me long to finish aggismennt of killing 1K steel dragons for example.