The beginning has a job to do, and the length depends upon how big the job is. The job is bigger if the Normal World is unfamiliar to the reader.
The job, in essence, is to lay the cultural, physical (e.g. does magic exist, is flying possible by any means) and emotional foundations for the decisions and actions that are to come.
The beginning must also contain whatever "inciting event" creates the main story problem for the MC. Typically these are unjustified: You note their homeland is invaded, but why the invaders decided to do this is not detailed.
(Or maybe they did it because of drought in their own lands: Why did the drought occur? Even if you have an answer for that, eventually you come down to something that just happened, like a lightning bolt after a dry summer that started a fire and burned all their crops, or you come down to a motivation like greed or a desire to survive that needs no further explanation.)
In film, where we have a very constrained amount of time to present a story (apparently based on the average bladder size), you get 5% to 10% of the total time to "establish" the MC and the world they live in (e.g. the time period, culture, the existence of magical or sci-fi gadgets or space travel, etc), and you are expected to introduce the main conflict within the first 20% to 30% of the time. Novels often turn out to have similar break points, that is the structure of the First Act. (Here is a useful discussion of this).
These are not concrete rules; but they are grounded in human psychology, or really they are a distillation of thousands of years of human storytelling, of millions of stories, and figuring out what works in popular stories and what did not work in unpopular stories (at least in recent decades we have many commercial failures to analyze, in print, TV, and movies).
The understanding of the world is an investment. If we invest the time and imagination to follow two hundred pages of setup, and then the main problem is introduced and solved in the last fifty pages, we don't feel like the payoff was worth the investment in understanding all of this.
Here is a thought experiment: Suppose Tolkien wrote 200 pages telling us all about the shire, introducing a lot of hobbits, how they live their lives, have romances, get married, have and raise their children, their religious ideas and politics. Complete with a series of arguments and disagreements, so there is some conflict to keep the story going. After 200 pages of this, we are introduced to the villain: A magical dragon! It killed brave Kevin! In the next twenty pages, the characters put aside their philosophical differences and petty grudges, band together, and fight and kill the dragon. they saved the world! Lost a few fighters, but they build a shrine, and we see their lives continue.
This probably would not be that popular a story. There is far too much setup, and not enough payoff, and it would seem (to the reader) unfair.
Your setup can be longer if the world needs a lot of explanation, but you should aim for the 20% level, and not exceed the 30% level for total length.
I don't know how you format your pages, but I presume you are using an editor that can count words. The typical length of a first novel is 100,000 words; but many are shorter. Some writing instructors recommend 75,000 words; JK Rowling's first Harry Potter novel (Sorcerer's Stone) was 76,944 words. In paperback this came to 384 pages; so 200 words per page.
The inciting incident in that book is the moment Harry turns eleven, the very second: Hagrid is introduced, there is some conflict but Harry is going to Wizard's school, end of story. In paperback, that is the scene of Chapter Four, page 57, 15% of the way through the novel (14.87%). Before that, we see Harry's 'normal world'.
The Transition to Act II is when Harry enters his new world, around page 117 (30%), boarding the train at Platform 9 3/4.
Your Ordinary world is similar. You have introductions to do; and you can explain SOME things later as they are encountered, but you must also balance the investment of the reader and their expectations. They expect things to happen, so if you run on too long, if they keep turning pages and they still feel like they are in setup mode a third of the way through your story, you are going to lose them.