It depends, really. Some prefer to schedule the the test from day one in order to stay focused and motivated. Other take a more conservative stance and schedule the test maybe a month ahead.
I chose the latter because it was pointless for me to schedule the test until I felt comfortable with the material. Plus, there are always unforeseen events that may force you to reschedule. You may even forget about the date if you select too far in the future. Scheduling later lessens those problems to some degree. However, there is one major drawback; if you wait too long to pick a date, then it might not be available. I had to wait over one week for the next opening! This can complicate your efforts to adhere to admissions deadlines.
There's no right answer here, so continue to ask around and find out what others have to say. After that you will know what feels best for you.