This question was recently asked in the comments thread on another post. Like many simple questions, the answer is a bit complicated. The short answer is that, as an academic, you never stop reviewing the literature because you need to be aware of the latest developments in your field and what your competitors are doing. I don't think this sufficiently answers the question though. As a PhD student aiming to submit a thesis there has to be a cutoff point somewhere; a point where you stop adding new content and consolidate what you have into a submissible form. There also needs to be some way of filtering and prioritising literature and deciding which references to follow up.
The Literature Review | A Complete Step-by-Step Guide
The purpose of a literature review is to highlight a void in the research that your study will fill. The literature review answers why you should conduct your research. To answer the why behind your study, find and analyze other studies that address similar research questions, or studies that address your research question on a different level. Discuss and present your research question and how the answer you discover will fill in another piece of the puzzle in your field. Your university library system should provide you with a login that may give you free access to articles you would normally have to pay for through these databases.
In essence, a literature review identifies, evaluates and synthesises the relevant literature within a particular field of research. It illuminates how knowledge has evolved within the field, highlighting what has already been done, what is generally accepted, what is emerging and what is the current state of thinking on the topic. In addition, within research-based texts such as a Doctoral thesis, a literature review identifies a research gap i. However, your literature review does not need to be inclusive of every article and book that has been written on your topic because that will be too broad.
In this post, I'm going to guide you through how to write a literature review on any topic from scratch, even if you haven't read a single paper yet. It can be as broad as you like because this is just a starting point. If you are still picking your specific topic for your PhD, that's fine, but you should at least know roughly what area you want to explore. A quick google scholar search for your subject area could turn up as many as 1 million results.