Well, that depends. You will understand that this, for example, depends on your earlier education and how much time you can dedicate to learning.
We publish 3 study plans to help students in various situations get all set for the GED test. We have plans for 1-month, 3-months, and 6-months. Three months is a good solid chunk of time to study for the GED test. This is the interval that many students use, but we have different plans to focus on different needs of different students. One month is not much time to learn everything that's on the GED test, so that study schedule has a very intense pace, and for those who need to learn basics, we offer a six-months plan.
Beware: our study plans refer to the preparation for one subject test. This is also our recommended advice: study just for one subject at the time. Then rinse and repeat.
Bear in mind that you can take one of the four GED subtests (modules) at a time within a two-year time frame.
It is a good idea to take a GED practice test to find out how far you are in your preparation to sit for the actual GED exam.