It varies from person to person. Some people learn quickly, while others take a considerable amount of time to prepare for the same. However, it is believed that three to six weeks should be sufficient, depending on your English language ability. 

IELTS has a definite pattern and the best way to understand the pattern of the test is by taking as many mock tests as you can. IDP provides a plethora of online practice material as well as a book will be sent to the address provided during the registration. The book itself contains 4-6 full practice test of comparable difficulty. Strengths and weakness may vary from people to people in the section-wise of reading and listening section marks.

For listening, Don't hesitate to watch TV Series with subtitles initially and make sure you stop using them when you feel confident. It is advised if you have a lot of time for preparation. If you want to see a drastic improvement, then spend time on practising tests and reviewing the answers. 

The solution will be the same for reading section too. Practising consistently until you can cross 30 plus correct answers mark. Speaking is where you can score by some easy hacks like using synonyms to show your vocabulary, pronunciation, focusing on grammatical range and accuracy etc. Watching TV series of English can help you with this. 

Writing has some definite patterns to follow. Remember, you should avoid spelling, grammar, and punctuation errors. Use different words as possible without losing the coherence of the essays.

All the best!