If you want to learn Jira then first figure out want exactly you want to learn. Do you want to learn Jira to use it or to administer it? In both the cases ask your self do you have previous experience in another tool or are you a total beginner? In my opinion there is no definite answer to this question.
Depending upon your previous experience I think usually it take couple of days to learn Jira, especially if you go through a training course. I have trained thousands of people to become Jira Administrator. Most of the training sessions I did were for corporate clients who want to train their employee to not only learn how to use Jira but also equip them to administer Jira. Also when clients moved from legacy tool to Jira they had to train their developers, manager and administrators to learn Jira as well. Although you can learn many things related to customisation in couple of days but if you are a consultant like me who help others, or want to perform upgrades, migrations, writing scripts and developing apps, it can take years.
I hope this bog post helps. Also, I have been writing this epic blog post to consolidate all the resources that will help you in your learning journey: Start your Jira learning along with plenty of my courses, all free ;)
If you need any help or support I might be able to help you if you raise a ticket for me :)