B school deadlines and information.

Apps generally open up during the summer (June/July time frame). You want to take the GMAT with enough time for the GMAC to submit your official score before the deadline. Round 1 is typically Oct 1, Round two is typically Dec (something 1, 15, etc.), Round three is generally in March. If you take the rolling admissions like INSEAD (or 1 year Jan begin INSEAD), you will need to check the INSEAD schedule.

Some have said it's fine it your score is submitted after the deadline (as the schools validate info if the school decides to move forward with your app), but I wouldn't risk it.

If you want to play it safe with a timeline, take the GMAT with a minimum of 15 days before the deadline. The GMAC says "up to 20 days for official score release", but it's usually much faster than this.