Methodology and Literature review in Masters dissertation
Defining such a number implies that you adjust your scientific work so that it fits into a fixed document breakdown. There are many arguments against such an approach, but since you asked for the number, here are some ideas for how to get it: 

Solution 1: Talk to your supervisor.

He or she is the one deciding about your grade, so it would be smart to follow their opinion about the structuring and divisioning of your thesis parts.

Solution 2: Find examples.

Most universities make masters theses public, either on the homepage or in their library. Go through some of them and figure out how other people structured their document. Use their work as guiding example.

Solution 3: Write a paper.

A masters thesis is something like your first scientific publication, so you can structure it as a conference paper:


Introduction + problem statement + methodology (15%)
Related work / literature study (25%)
Your approach and experiments (35%)
Your results and conclusion (25%)


The weighting will highly vary, depending on the problem class. Pure survey papers have no experiments, so you spend way more space for the literature study part. Engineering papers only have a short related work section, so that there is more space for the approach / architecture section.