Or is it given after completing the main quest?

Crafting it is one way or find it off npc's (not sure about that last part cuz i crafted mine hehe)

It is not given to you at the end of the main quest however.

Thank you! Do it take a long time to get to have 100 in smithing?

*thinks*

Honestly not totally. Once I was able to do dwarven crafting it makes it a LOT easier since finding pieces of dwarven metal is as easy as finding potions if you go to their old buildings. It's how me and my friend did it.

You can also purchase the skill from the guy in Riften that does crafting (I really can't remember his name off the top of my head). He's a master so he should be able to train you pretty high up but only 5 ranks per level of course.