How much of an academic presentation should be spent on preliminaries?
There isn't really an easy answer to this; it comes with experience.  Over time you get to develop a sense of "standard things" that you can expect your audience will know.  Sometimes you'll err in one direction or another, and that's just life.  And you can't expect to please your entire audience with every part of your talk.  No matter what you do, some parts will be trivial to some listeners, and other parts will be inaccessible to others.

In the meantime, if you're a student, you have an advisor, and that's a source of experience for you to draw on.  Go over the talk in advance with your advisor, and ask for their advice as to whether you have too much or too little detail on preliminaries.

One very rough rule of thumb is to assume your audience knows any topic that would be covered in a typical first-year graduate course in your field, particularly if it's the sort of course that everyone would be expected to take.