The answer to your query is subjective, that is it depends on you, how much time are you able to and ready to spend on learning Tableau? Well, Tableau is very easy to learn and use, so if you devote a healthy amount of time, then you could learn Tableau within a month.
As mentioned above, it varies from one individual to another, some might take 2 months or even 6 months to complete. Anyway, check out our tableau tutorial which might help you in getting started.
Then once you are confident enough, enroll for a good course on Tableau, and for that, I would recommend you to take up the Tableau certification course from Intellipaat.
Also, watch our YouTube video on Tableau training for beginners if you want to know more.