Really anytime. The test isn't that hard. But heres the kicker....The part of the test where they say there are no wrong answers...Don't believe a word of it. You can take it more than once. Have you been studying the test prep?
I'm not trying to misrepresent myself. I am not a navy pilot. I am a civie. But I did complete the ASTB successfully. I took mine in my sophomore year.