The IELTS writing test takes about an hour. It is different for academic and general training. In both modules you have to complete two tasks. In the academic writing module you will be given 20 minutes to write a letter or report and 40 minutes to write an essay. The report or letter must be of at least 150 words. The essay must be of 250 words.
In the general writing module, the first task is always a letter (150 words) and the second task an essay (250 words).
In your essay you should be able to organize your thoughts clearly and provide a written argument. You should also be able to provide examples to support your argument. Note that in both modules you have to write on the topic you are given. There is no choice.