You must click on next once you are done speaking your answer. You need not wait for the recording to complete automatically.
Neverthless even if you don't hit the next button, the computer will stop recording after either a silence of 3 seconds or if the recording timer reaches completion.
But in no case you will be awarded zero point if you don't hit the next button.
We would recommend you to hit the next button as it work on speech to text giving gap in your speech will lead to Oral fluency issues.
So @bestptecoaching we unconditionally recommond you to click on next button the moment you are done with answering any speaking question type i.e Read Aloud, Repeat Sentence, Describe Image, Retell Lecture, Answer Short questions.