I've been trying to get into a BJCP exam for over a year now.
The big bottleneck is the lack of graders. That's why there's a limit on the number of tests that can be offered each month. You have to be a National or better judge (meaning only about 10% of the judges out there qualify) and, currently, grading is a time-consuming process that burns out lots of people.
It's been my experience that it takes at least a year to get on the schedule, sometimes more. You're actually doing pretty well to get an exam slot 18 months out. Other people have given good advice about getting on a waiting list.
I'll add that you don't need to be a BJCP-ranked judge in order to judge, and that you should probably get some judging experience before you take the exam. Contact your friendly local homebrew club, tell them that you're interested in judging and ask what resources they have to help you. There are online sources which can help you prep for the exam, but it really helps if you can judge beer with experienced judges. If you're really serious, start a BJCP study session/training class with other folks who are interested in becoming judges.
Also, don't let yourself get stuck in the cellar at your next competition (although a good cellarman is a blessing when running a big competition). If you can't judge, at least steward. That way you can see how judges work. If the judges are on their game, they'll also let you drink with them and might use the session as a tutoring session.