The IELTS Listening test takes about half an hour or 30 minutes. The 20 minutes is for listening the audio cd as well as answering the questions simultaneously, don't transfer it yet to your answer sheet as you have a lot of time to do this after the audio cd stops because you will be given 10 minutes to transfer all of the answers that you have written on the booklet.
Concentrate on listening and working out your vocabulary in this sub test is your vital key in getting a high band score (because sometimes there would be sound distractions from the outside and you might miss it if you can't focus.) as you would only hear the Audio cd once.
Listening tests has 40 questions overall and it has 4 sections and it is going to be more difficult as it progresses.
Section 1 and 2 are usually based on life or social situations like travel, new environment, ordering a new things, phone conversations and at least there are 2 speakers are being heard in the conversation but section 2 has only 1 person speaking because it is usually in an informative way like describing a place or reporting. While section 3 and 4 is based on education like group discussions, or a class presentation or feedback, describing a new environment. Section 4 in particular is more on a lecture or about anything that is in general interest.
When in times that you did not clearly heard a certain part of the conversation and has missed a question proceed to the next question as you can also miss that when your figuring out what you have missed. You can get check it back later when you have finished transferring your answers to the answer sheet and make an intelligent guess. And make sure every number is answered never leave it blank just make a guess or have that word filled that makes appropriate word to make it complete or just use a synonym (this maybe applicable on other questions but in numbers you cannot make an alternative for that so the best thing is to listen carefully).