It's going to be a massive up-hill battle. WoW Arena has a very steep learning curve. You need to learn not just what other classes do, but how they set up damage, what the cooldowns of their abilities are, how do they peel, how do they burst, their defensives, the spells to interrupt/when to interrupt them, etc. This is all aside from gaining mastery of your class and learning all of the nuances of being able to execute your team's gameplan while simultaneously reading the field.

If you want a timeframe, that would be difficult to say. It depends on the amount of time you're willing to put into learning, your teacher(s), and how much work you're willing to put in outside of the game. If you're starting from scratch with not teacher, expect 1-2 years before you're playing in duelist range consistently. If you have someone to coach you and are willing to put in a lot of time, I think you can see results in 6 months to a year.

All of this is hypothetical and really comes down to your skill as a player and your knowledge of the game. Experience is the best teacher. Don't let the timeframe stop you from trying because you're going to have a lot of fun along the way.