It really depends. Here’s my timeline for your reference (abt my background: I study language in university and my TOEFL scores are consistently at or above 110)
Late May: Start preparation, but I did that along with my internship and my driving license test - so the progress was slow.
Aug: My internship was finished and I was wholly dedicated to GRE preparation. The biggest task was to master a large vocabulary - for me that could only be done in a short time as I would forget them when time passed... 10 days before the test, I did a mock exam and my verbal score was 156. Sincere quant test was ok for me, I dedicated most of my time memorizing the vocabulary, going through the vocabulary list (要你命3000, a Chinese reference book for GRE vocabulary for about 3-5 times)
Aug 21: I took the exam with 165 for verbal and 166 for quant.
It really varies from person to person as to the preparation time as people have different backgrounds and learning habits. Still, getting much of the work done in the last 3 wks is possible but very VERY Suffering! Therefore, start early and leave enough time is always recommended 🙂