How soon should you start taking full tests before real GMAT

I disagree.
Just don't check your answers, and after two weeks or so, you won't remember a thing

And what would be the point of taking a practice test without checking the answers?

Seriously, to the OP...don't waste a test. Start studying with the practice questions and identify your weaknesses that way. Do 10 or so questions for each type and review your errors. Come here for help if you're stumped. Take a test after you've done some more practice, then revise your prep strategy after you see the results. Just be sure not to neglect areas that are average to strong so that you don't blow those on your tests!

If verbal is a weakness, I would highly recommend getting Verbal Workout from Princeton Review. The strategies are great. Altho if you have their Cracking the GMAT book, you'll have the strategies along with several tests. I found the answer explanations to be very helpful. The workbook just has more practice questions. Good supplement if you finish the OG and still need more practice.