First off, don't take it if you aren't ready. You get to take it on your schedule, not your OSO's.
Second, you have plenty of time to study. I took mine on a Monday and only studied the Fri, Sat, & Sun before. I used the Arco book and gouge listed on this site. I pulled out a 8/8/8 56. It wasn't anything too terrible.
Finally, If you do not do well on your first ASTB you may take it 30 days later, and if you do poorly a 2nd time you will have to wait an additional 90 days before you are able to take it for the 3rd and last time. Your scores are completely replaced each time you take it. That means if you do better, then you keep the better score. That also means if you do worse, then you keep the worse scores.