2-3 years of devoted, full-time is probably maybe enough time to begin to familiarize yourself with the basics of a single field of historical study. I've been deep diving in pulp studies for about eight years, published several books and a number of articles and essays on the subject, and I still learn new things every day.

It is important to understand what a college degree is and isn't. It is direct evidence that you completed a course of academic instruction on a field of study, and an ability to complete work that satisfies the requirements of the institution that you have absorbed a enough of the material to put it into practice. Higher degrees reflect an ever-deepening level of knowledge within the discipline and specific areas of study within that discipline. It's not that a PhD in history necessarily knows more about the literature than an independent scholar without a degree, but the PhD has proof that they passed the examinations and thesis or dissertation defenses to show that they can interact with the material in an advanced way - because understanding history is about more than just reciting facts or reading primary sources though those are important.

It's about being able to recognize what is and isn't in the sources, what the issues are with the field and how other scholars have grappled with them, the historiography of a particular area of study, what we know and don't know and suspect, and how that has been communicated to students, scholars, and the public. To be able to competently analyze and criticize new and old works on the subject, and to create your own contributions to the overall literature.

You don't necessarily need a degree to do that, and there is not going to be any singular plan or course that will get you to that point. Even a graduate degree will only give you as much as you put into it, and the learning never stops after you get your diploma. So if you're feeling passionate about history and want to dig deeper...you have to start out by understanding that for the people who are really dedicated, this isn't just a 9-5 job or a hobby. It's an avocation. Part of who they are and what they do. It might not be their entire life or even the defining trait of their personality, but it's going to be a big commitment in time, effort, and interest. Many historians are passionate about history; they devote their lives to it.

So before you start applying to schools or binging on documentaries...sit down and really think about what you want to do with your study of history. If you actually want to work as a history professor or at a museum, you might need a degree; if you want to write a book, you don't need any qualifications but you'll want to begin planning things out and doing the basic research first. Above all else, you're going to want to zero in on what area of history you're most interested or passionate about - while lots of people have broad historical interests, the whole scope of human history is generally too broad to encompass within a reasonable human lifespan, unless you're taking an anthropological approach of humans as a species.

So before you dive in...figure out what you actually want. Then you can start making a plan to achieve that goal. If you wanted to read the Iliad in the original Greek, for example, you'd need to first figure out what the history of the text is, what manuscripts and variations exist in what traditions (and if they're available), identify the correct languages and then learn to read those. Some of which might be easier with courses of study, some of which you can do on your own.