Well, that depends. Generally, if you will study for one hour every week, you can be sufficiently prepared in 5 to 6 months.
Then again, if you are ready to study 2 x 30 minutes per day, you could be all set in about 2-3 months. You will understand that this all depends as well on your earlier education.
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.
Here are the free practice tests:
Here you can prepare for the GED exam for free: