Your question is quite tricky as the answer may vary from one person to another. As you know, IELTS is an English language proficiency test, and you need proper practise to nail it. Generally, 5 to 6 weeks is the average time duration required for IELTS preparation. But, if you already have a good command over this language and familiar with the exam, then it's possible to prepare in 15 days.

In reading, you would be tested for reading the comprehension, which means grasping the power or understanding the written word. While preparing for the exam, you might need to do a lot of reading. It could be news articles, scientific analysis or observation by academicians of current issues and matters like the latest application of artificial intelligence or development of waste management systems in the UK. 

In listening, it is all about how well you can understand the English language. Different English accents and the matter of which is being spoken of in a conversation etc.

Coming to writing, it is probably the toughest sections in the IELTS as it tests several parameters like grammar, sentence construction, essay structure etc. 

In speaking, it is the test of one's ability to hold the conversation or like merely explaining a complex idea. So, it is all about finding the right tutor who can guide you based on your strengths and weaknesses. Keep in mind; an experienced IELTS trainer can assess your readiness for the speaking part. 

If you want to score above band 8 in IELTS, then you need to start your preparation as early as possible. All the best!