the straightforward answer to how long should a literature review be is roughly 20 to 30 percent of your total dissertation word count — so around 2,000 to 3,000 words for a 10,000 word dissertation and 3,000 to 6,000 words at masters level. always check your module handbook first though because your university guidelines will always be more accurate than any general rule you find online.
what matters far more than hitting a specific number is whether your how long should a literature review be target is filled with genuinely critical and analytical writing rather than just summaries of what various authors said. get the length roughly right and then focus entirely on making every single word count