Hi raffaytarar,
This is the 'busy' time of year for GMAT Testing Centers, so you will likely have to plan far in advance to get the exact Test Day and Time that you're interested in. That having been said, some Centers are less busy than others and could have open 'spots' available in the short term (and Test Takers sometimes pay to reschedule their Exams - when those moves are made, spots DO open up at those Centers).
So, to answer your question, you COULD conceivably book your Test Date just a couple of days in advance. Considering the importance of the GMAT score to your application(s) though, you'll probably want to book your appointment far in advance.
GMAT assassins aren't born, they're made,
Rich