Your prep for TOEFL has nothing to do with your prep for the GMAT. Most people prepare for 2-3 months studying a coupla hours a day and about 5-6 on the weekend. How long you prepare is entirely up to you. Depending on your level of proficiency you might want to take more than 3months. Some people start preparing and then decide that based on their prep, it'll take them another X months to be ready and then book a date while some book a date 4months in advance and start preparing. Remember that postpoing the exam costs $50, you can't postpone a week before the exam and the exam itself costs $250 so when you book a date is entirely up to you.