It depends on your previous skills and education. If you've got a good math background and your native language is english.. 1 month might be fine. If your shaky in math or not a native speaker I would argue for more time.
Also, of course, your target score would be a huge factor to consider.
The advice I gave my friend who just wanted to do it so he could hurry along to a MBA program was 1 month. That'll give you enough time to learn some of the tricks of the test and review concepts you haven't seen in awhile.