Hi 12Pranav,
If you are asking about the two GMAT formats (the traditional Test Center format and the At-home GMAT) because you want to retest quickly after earning a lower-than-expected Official Score, then you might want to hold off on retesting too soon. The GMAT is a consistent, predictable Exam, but if you scored lower than expected, then there are likely some 'inefficiencies' in your approach to the Exam that won't improve until you put in the necessary time to learn, practice and master some specific Quant and Verbal Tactics.
To answer your immediate question though, you do NOT have to wait 16 days (after taking the GMAT at a Test Center) to schedule an At-home GMAT and vice-versa. However, the two formats are notably different from one another - and it takes time to properly train for one format (especially if you have been training to face the other).
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) Have you taken the Official GMAT (and if you have, then how did you score - including the Quant and Verbal Scaled Scores?)?
2) How long have you studied in total? How many hours do you typically study each week?
3) What study materials have you used so far?
4) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
5) What is your overall goal score?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich