Stephen King said, "If you don't have time to read, you don't have the time (or the tools) to write. Simple as that." This is as true for academic writing as it is for creative writing. The literature review that you will do for your dissertation positions you in the knowledge of your area of study. You must be fully immersed in that in order to do a dissertation.
Having been on a few doctoral committees at this point, I can tell you this is a place where I see people struggle a lot. In fact, the literature review was perhaps the most difficult section of the dissertation for me as a doctoral student. Here are a few general guidelines to consider.
1. READ EVERYTHING. Go to the old stuff and read everything that has been published through today. You are expected to have done this. In one of my classes in my doctoral program someone asked, "When will I know if I have read enough?" The professor responded, "When you start finding the same work over and over again. You will see work referenced in an article or a dissertation and you will say to yourself, 'Oh! I've read that.' When that starts happening all the time, then you have read enough."
Now, just because you read it doesn't mean you will cite it in your proposal or dissertation. But you will know enough to know what the seminal works are and what pieces align most closely with your work.
2. Talk to subject matter experts about what and whom you should be reading. Hopefully, you have a committee member who knows something about what you're doing for your dissertation. Ask them to steer you in the right direction (but to neither do the lit review for you nor hand-hold you through every step of the process).
3. READ DISSERTATIONS. Dissertations on similar or related topics are perhaps one of the best ways to start your literature review. Read their reference lists. Read their lit review sections. What work are they citing?
4. Connect the literature back to your research question / study. How does what you're reading connect to your study? How does your study add to what the literature you are reading has done. Do this as you read AND as you write your literature review chapter.
Keep up the good (and hard) work!
P.S. Below is a chart compiled by Dr. Maureen Wilson through CSPTalk on texts related to literature reviews. Thought you might find it helpful.