I would suggest to attempt the 2nd test of GMATPrep as well, preferably after sometime when you think you have acquired enough knowledge. Then you can do some analysis on where you went wrong and spend some more time on practicing the problematic area. You can reuse the software again, as it randomly picks the question and it may not be same during 2nd or 3rd attempt.
If you give the test at the end, you won't have any time to correct yourself.
Btw, regarding your question on how much time is required for preparation, thats a very relative term. You can't acquire ALL the grammar skills or reading skills even in years. So practice, practice and practice as much as you can. Most of the questions in GMAT are tricky, rather than being time consuming. For example, in quant, if someone just mug up all the theorems and formulas, he may not perform well in gmat. Its all about one's IQ level. Practicing and brushing up concepts certainly helps. In verbal, specially for non-native speakers, the reading skill is most difficult problem. Atleast thats true in my case. But again, improving reading skills is much difficult than learning anything else.
In traditional exam, you can say that I have completed my syllabus book 4 times, and so I'll perform very good at exams, as all the questions would be from that specific book only. Whereas in gmat, its all about timing, concentration, intelligence, keeping your mind stable for long hrs, and of course the understanding of basic concepts.
Disclaimer: I haven't attempted actual gmat yet.