I don't think that there's an easy answer to your question. Learning to use a screen reader is similar to learning a new language, so the amount of time that it will take is going to depend heavily on how immersive your screen-reader usage is as well as other things, like the quality and quantity of any training you've taken, your natural ability to learn, etc.
Keep in mind that using a screen reader is just plain difficult. You'll get better, but it's always more difficult than being able to visually read a screen. Understand that it's going to take longer to do most things, even after you become proficient.
I'm not sure what you've tried already, but there are a few things that may be helpful for you:
- Adjust the contrast. Many applications and OS have a "high-contrast" mode that can sometimes be helpful for people who have sensitivity to light (although sometimes it's not). Using a black background can significantly improve experiences for some people. Try setting the text to white first and then reducing the contrast as needed so that your eyes don't hurt as much but you can still read the text.
- Try different screen readers. NVDA is pretty decent, but Voiceover on Mac and JAWS are also very good. You may find that you have a personal preference for one over another. To me, Voiceover is much easier to use, but I find that it's not as full-featured.
- Try different browsers. You may or may not be aware but the browser that you use makes a huge difference in what your screen reader will or won't announce. Historically, IE and FireFox have been most popular in the accessibility community, but with IE mostly being obsolete at this point, I wouldn't recommend it.
