There is no set time required to prepare for IELTS as it completely depends on your current English language skills. So, firstly you should check your current proficiency by taking any of the basic grammar tests available online. It will give you an almost accurate result to help you decide further.
Basic Level User
For a person whose English language skills are very basic, more time will be required to reach a level where he or she could score a decent band. You might need to practice for 3-4 months to achieve it but it can be done, so don’t lose hope and make a plan to study and practice.
Intermediate Level User
It is always advised that you give at least practice for one and a half or two months before taking the test if you have an intermediate level of proficiency in English. You might be able to score good in two or three modules but not all equally. Hence, it is important that you practice all of them but focus more on the one in which you are weak. For that, you need to know what your weak module is so that you give it more time.
Advanced Level User
If you are already a good user of the language then too practice is required, so do not skip it, practice at least for 4 weeks and then book the Test. Sometimes people make a lot of silly mistakes due to overconfidence which will be really dangerous and this generally happens if you are good in the language skills as then you take things light and do not pay enough attention to it. Stop yourself from doing this, practice and then take the test.
Scoring High
If your aim is to score high in IELTS as per your application or you want to do that for your own satisfaction, it is essential that you practice a lot even if you are an above average user of the language. Your focus should be on improving vocabulary and showing good range of sentence structures including simple, compound and complex sentences, using popular idioms and phrases in Speaking as well as Writing.
Scoring 9 bands is quite easy in Listening, you just need to listen actively to the recording as it is all about concentration, so do not think about anything else while listening to the recording.
Similarly, ignoring distractions and focusing on the text of the passage will help you to score better in Reading module. Pay more attention to the use of synonyms along with rephrasing and you will be able to perform well in the Reading test.