Hi Andrew.
The answer really depends on what works for you personally and how you prepare.
Some people would likely find preparing over the course of 10 months or more dragging things out or might have difficulty retaining what they learned for that long of a period. I'm certainly not saying that preparing for 9 to 12 months would not work. I'm more saying that some people might find it way less than ideal.
For others, whether they prepared for 6 months or 10 months wouldn't make much difference.
You can make it make less difference by thoroughly learning the hows and whys of topics so that you remember them because you have logical frameworks supporting your memory.
In any case, preparing over a long period can work fine as long as you stick with it, learn topics thoroughly, and review topics from time to time.
On another note, you may not need that much time anyway. After all, it could be that you already have much of the knowledge and many of the skills necessary for achieving your target score.
You can get a better sense of how much time you'll need by taking an official practice test after preparing some and seeing how you do.
For some tips on how best to prepare to achieve your target score, see this post.
How to Score 705+ on the GMAT Focus Edition