After a candidate has completed the ASVAB they must wait one calendar month before retaking the exam. An additional calendar month must pass before retesting a second time. Six calendar months must pass before retaking the test a third time. The scores received from the ASVAB may be used for enlistment for up to two years from the initial test date. 

The ASVAB was created in 1968. By 1976, all branches of the military began using this test. In 2002, the test underwent many revisions, but its main goal of gauging a person’s basic skills remained the same. Today, there is a computerized version of the test as well as a written version. The Department of Defense developed this test and it’s taken by students in thousands of schools across the country. It is also given at Military Entrance Processing Stations (MEPS).

You are tested in four critical areas — Arithmetic Reasoning, Word Knowledge, Paragraph Comprehension and Mathematics Knowledge (see below) — these scores count towards your Armed Forces Qualifying Test (AFQT) score. The AFQT score determines whether you’re qualified to enlist in the U.S. military. Your scores in the other areas of the ASVAB determine how qualified you are for certain military specialties. Score high, and your chances of getting the specialty/job you want increase. The better score you get, the better chance you have of pursuing the specialty or field that you desire.
The first step that one will take after deciding to pursue a career in the military is taking the Armed Service Vocational Aptitude Battery test. This is more commonly referred to as the ASVAB. The ASVAB not only determines whether or not a candidate is qualified to serve in the armed forces but also shows which specific job they have the most aptitude for.
I have already served in the military years ago. But I wondered if there was a primer that might have helped me improve my test scores. I think this book would help. Unfortunately it's hard to learn enough from a book to do well on the math and reading comprehension of the ASBAB test. Study hard in math and English class in high school. It's hard to cram from a book to pass a comprehensive test that covers many topics. I think it may give you a few extra points, however.
If you didn't earn a score that was high enough to be accepted to your chosen branch of the military, or if you didn't earn a line score high enough to qualify for a desired career option, then you may retake the ASVAB. You must wait a full month before you can take the ASVAB a second time. If you wish to take the test a third time, you must wait another month from the date of your second attempt. After the third attempt, there is a waiting period of 6 months before you can take the test again.
During the war, significant advances were made in military communications through increased use of radio, military intelligence through use of the radar, and in military medicine through use of penicillin, while in the air, the guided missile, jet aircraft, and helicopters were seen for the first time. Perhaps the most infamous of all military technologies was the creation of the atomic bomb, although the exact effects of its radiation were unknown until the early 1950s. Far greater use of military vehicles had finally eliminated the cavalry from the military force structure.
If you are enlisting in the military, good - a patriotic journey, where you can build skills and see the world. However there is one thing your recruiter didn't tell you. Your ability to get into a department that you want, depends on how well you can take a quiz - just like you did with the SAT, or whatever those test are called in current times. Those test asked those really stupid questions about the cost of ice cream if a train leaves New York at 4:00 and it is 20 degrees on Colorado. This book will teach you how to answer those ridiculous questions that WILL control your destiny. Get a copy and memorize it... then serving your country will lead to a great career.
Figure out which areas to focus on based on your career goals. If you’re not interested in a job requiring a score on the Mechanical Comprehension subtest, you don’t need to worry about doing well on that subtest. If you don’t need to worry about mechanics, don’t bother preparing for that section. Spend the time on Word Knowledge or Arithmetic Reasoning.

Mathematics Knowledge tests the ability to solve problems by applying knowledge of mathematical concepts and applications. The problems focus on concepts and algorithms, and involve number theory, numeration, algebraic operations and equations, geometry, measurement and probability. Mathematics Knowledge is one factor that characterizes mathematics comprehension; it also assesses logical thinking.