1. Take an official mock exam.
Students often ask “How much time do I need to prepare for the GRE?” The shortest and only absolutely accurate way of finding this out is by attempting a mock ETS examination. I tell this to students and ask them if they intend to appear to find out, more often than not the answer is no. Reason? Taking mock exam scares them. I would say it’s a bitter pill to swallow, but it’s absolutely worth it. There is no other absolutely accurate way. That said, there are some other ways you can use to determine how much time you need for preparation but they are not as accurate.
Firstly, most students take somewhere between 8-12 weeks to prepare for the GRE. If you have access to quality preparatory material, a defined strategy and can devote 1-2 hours every day, 8 to 12 weeks should be ample time. This is not to say that all students can prepare for the test in 2 to 3 months, some might take up to 6 months or even a year while others may ace the exam in a month.
Engineering or Finance background
In general, if you have a sound mathematical base and read frequently, you need the aforementioned time. If you have a strong mathematical base but are a non-native/ not good at verbal reasoning, you may want to spend some time improving reading comprehension and understanding text completion and vice versa.
Another important factor is your occupation, are you a student or a Maths teacher? Or are you an engineer working at an oil field since the last 3 years, usually recent graduates and students tend to take less time because they are familiar with the concepts and have the stamina to study. Again, if you are fortunate to have a job that requires a lot of reading and calculations, you might need less time but most employees take longer to prepare the GRE.
4. Target Score.
How long you need to prepare for GRE also depends on what your target score is, your target score is generally a few points more than the average GRE score required for your target school. You can find the average GRE score on their website or, if the data is not available, by emailing them.
If you are aiming for a competitive program at a top-tier university that asks for a great Quant score, you should a few weeks on sharpening your Mathematical skills even if they are already good. On the other hand, if you are aiming for a Masters in political sciences, you need to put in less effort if you are good at Quant already.
In short, the time required to prepare for GRE depends on your initial score and your aim. Students with great GRE scores often either put 100+ hours or are good at Quantitative and Verbal skills even before they have started their prep. One simple way to calculate the number of weeks required is; estimate the number of hours you can dedicate each week and divide 120 by that estimate. Keep in mind that not all students require 120 hours, some require more, others less.
From my own experience, if someone has quality resources filtered out and can dedicate time, one can improve 10-15 points after 2 to 3 months effort. Generally, a good preparation strategy is to focus more on your weaknesses, and less on your strengths.