There is no rule for how often you should use them, but you should listen to how native speakers use idioms. If you listen they do not use them as often as you think. Online IELTS Coaching in East Delhi.If you overuse idioms you are going to sound very unnatural, you will make more mistakes and you could also lose coherence because you are focusing more on idioms than actually answering the question. Additionally, if you use lots of idioms the examiner will think you have simply learned lists of them rather than having good appropriate English speaking skills. Finally, it could also lead to your fluency suffering because you are thinking too much about idioms and this will cause you to hesitate and pause while you search for the latest idiom.