I wonder how hard (In your opinion) would it be for me to learn OpenGL?
Its almost impossible to give you the correct answer for this since we have no idea what your skill level is in programming, what your intelligence is, how well you follow through with things, and a number of other variables. If you don't understand the basics of C++ VERY well I would hold off on openGL till you do. But really your the only one that can answer this. Just stick with it and take your time, when you try and rush into the advance stuff before you have a good grounding with all the beginner stuff you will run into all sorts of trouble.
How much effort would it take to make a doom-type game?
Again depends on your skill level and how determined you are. I would advise you since you don't know any graphics programming (I assume) to start simple with 2D puzzle games, 2D platformers, 2D anything first. 3D games and especially FPS take a lot of time and effort to make a good one.
To put it in perspective most good 2D games built in higher level languages like C# and Java can take at least 3-5 months for very experienced programmers to make. Games like the Call of Duty series takes a year if not more to make with probably 500+ people putting in 40 hour work weeks. So again take it slow and start with little things first.
Again we don't know your level and we don't know how long it will take you to learn it. For some people it takes a few months to get the basics down for others a few weeks to get the basics. Everyone is different. What does it matter anyways? Just go out and start learning it and then soon or later you will know it.
To master anything in programming use the 10,000 hours rule. And even after them 10,000 hours spent on a subject you still have more to learn. You can just spend a set amount of time learning something in programming and expect to know it. It is constantly changing and you will be constantly learning new things.
I completely agree with everything Zereo has said. You have to answer this question for yourself.
If you don't know how long it'll take to learn something, invest a few hours (or days) skimming/reading tutorials to get a general idea. From there you should be able to get an idea of whether or not you feel you're ready for it, or how long you think it might take to get a full grasp of it.
It shouldn't matter then, just give it a go and start learning openGL from whatever resources you have available. Don't restrict yourself to just learning openGL. By this I mean if you are following a tutorial and you don't understand what a inline function does, stop for a second and research what inline function does and once you understand it go back to your tutorial where you left off. You will be doing this a lot trust me, just make sure that if you don't understand something that you research it and make sure you understand it before you move on instead of just saying oh well and skipping it because you think it won't matter.
This is how learning programming goes, there is no set path to follow, you have to be able to research things you don't know.
Another example is my personal experience with game design.
First I was like you and said well I want to learn how to make a killer game so I start learning DirectX, but I found out I couldn't even create a window until I knew at least a little bit about windows programming and the winAPI. So I took a break from directX and studied windows programming for a bit, once I had a good grounding on how to create a window I went back to directX. Then I found out that I needed to learn something else so I stopped once again with my directX tutorial and went and learned that and then finally went back to directX. This went on and on and on.
So the moral of this story is that you have to be flexible.
So my advice to you is start your doom game project and see where it takes you. I guarantee that if you stick with it till the end you will have a good understanding of how graphics programming, AI, user input , audio, and much more works.