Hi juniperevelyn.
While many people prepare for around three months, the amount of time it will take to get from 540 to 670 depends on a variety of things, including the reasons why you didn't get right answers to certain questions to your overall psychology.
For instance, if you are basically comfortable with the concepts that are incorporated into construction of GMAT questions and just need to train to get right answers, you may more quickly increase your score than you would were you to need to learn about many foundational concepts.
Also, some people just seem psychologically blocked from hitting their score goals, and dealing with such blocks and resolving things sufficiently to allow oneself to score high can sometimes take a fair amount of time.
So depending on your timeframe and your assessment of what you need to do to hit your score goal, it may make sense to start preparing some and see how things go before you take a month off. You may get the sense that you are going to need more than a month to learn and process everything you need to, or you may get the sense that a month is just right for getting to your score goal.
