The JLPT is divided into 5 different levels (although it used to be 4), in order to better categorise a person's overall level of Japanese (includes comprehension tests, although no oral and writing to this date). The levels are referred to as N1 to N5, with N1 attesting to the highest level of Japanese (fluency) and N5 for beginners.
The time allocated to each section varies depending on which level test you are taking, as well as the expectations. It is generally understood that you need 150 hours of study time, and know 100 kanji and 800 words of vocabulary to pass the N5, and over 900 hours of study time, 2,000 kanji and 10,000 words of vocabulary to pass the N1.
The test is currently broken down in the following format:
N1 – 170 min (Vocabulary/Grammar and Reading – 110 mins; Listening – 60 min)
N2 – 155 min (Vocabulary/Grammar and Reading – 105 mins; Listening – 50 min)
N3 – 140 min (Vocabulary – 30 min; Grammar and Reading – 60 min; Listening – 40 min)
N4 – 125 min (Vocabulary – 30 min; Grammar and Reading – 50 min; Listening – 35 min)
N5 – 105 min (Vocabulary – 25 min; Grammar and Reading – 40 min; Listening – 30 min)
The test is only offered twice a year, in July and in December, although some institutions may only offer it once a year. More information can be found at on the official JLPT website, including a list of all worldwide institutions (in English and in Japanese).