My answer is still that most people spend about 3 to 4 months studying for the test - but you still need to adjust accordingly for your specific situation. You want an especially high score (a 750 is crazy high). That may take more time. How much time do you have to study? If you can only study 5-10 hours a week, you may need more time. What is your starting score? Someone starting at a 500 is going to need more time than someone starting at a 600. Etc.
I also have to mention that 750 is so high, you really have to become a master of taking this test. Some people take 6 months or a year. Some people never get there. Years ago, when a 750 score was still 99th percentile, we had a few people who wanted to teach for us who would keep getting 740. We thought they were great teachers and tried to help them break over to 750 - gave them some free resources and pointers, that kind of thing. But some people just kept getting 740...so they never taught for us.
(Also: unless you're trying to teach for us, too, I hope you will be very happy if you get a 730 instead of a 750 and just go with it.
If you break a 700, the school is going to say, okay, he/she did very well on that requirement. Now let's scrutinize the rest of the application.)
Next, if you really do want to hit 750, then you are going to need most of the material in the Extra / advanced chapters, yes. You don't know which material will actually show up on the test, which means you do need to look at it all. And you will still have some weaknesses even at that level - you will still have to let some things go / you won't be good at everything. But you will need to be able to handle a lot of advanced material in order to hit that kind of score, even though you'll still get some wrong.