You can take the GMAT about a year before the expected entry into the business school. You will have enough time to work on processing the request. Also, when first discovered, not planned that within a month. Also, the result will come after 20 days.At minimum, you want to have the GMAT a month before the application deadline. It will take two to four weeks to get your official score, even if unofficial score immediately after the test. You can arrange for the test results must be sent directly to you and up to 5 schools in which you are applying. If you have a test in the vicinity of the application deadline, You May want to include the unofficial score report with your application.
Do GMAT only when you have prepared and are confident that you can do it.