I never recited New Concept English, but I do hear a lot of people recommending so.
Personally I believe that reciting can be one of the most effective way of learning English. If you are doing NCE, keep doing.
Although it seems fair that reciting both 3 and 4 would take probably one and a half year, you definitely do not need to wait that long before taking an IELTS, because taking an exam is different. It takes good English competence to excel in the exam, but bear in mind that excelling in exams doesn't mean excelling in reciting.
It can be very helpful. After I recite half of book3, both my listening and speaking skill has been improved. I haven't even started book 4 before taking IELTS, but it's already enough. BTW, you will get something called language feeling by reciting. However if you have the opportunity of communicating with the native English speaker directly, that's a different story.
NCE may be a systematic textbook for you to improve your grammer ability if your grammer is not so competent that makes negative effective in your communication.But reciting texts isn't a good way to master the original principle of English which contains two important parts:words and grammer.It means that once you overcome words and grammer of English,you will be an excellent speaker. What IELTS tests you is your abilities in speaking,reading and writing,not in reciting.So I don't recommend you recite any texts,to read texts loudly for several times instead,with purpose of remembering the important words and grammer in the text.