Hi fenjamin,
Many GMATers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so you've given yourself plenty of potential study time before your planned April Test Date. In addition, studying for the GMAT far in advance of when you will actually 'need' your Score is a smart choice (as it gives you the opportunity to focus on the GMAT for now).
Since it sounds like you're just beginning your studies, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can access 2 for free at
www.mba.com (and they come with some additional practice materials). If you want to do a bit more studying, so that you can familiarize yourself with the basic content and question types, then that's okay - but you shouldn't wait too long to take that initial CAT. That score will give us a good sense of your natural strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have that Score, you should report back here (or you can feel free to PM me directly) and we can come up with a study plan.
GMAT assassins aren't born, they're made,
Rich