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As you have known, HSK tests are divided into 6 levels. However, the speaking tests （口试） are separate. There are three levels of them: beginner, intermediate and advanced (初级、中级、高级). You can take them separately. I don't think you would have problem to take the advanced test if you are already fluent in speaking Chinese.
All the tests are made up of three parts. The beginner tests are about 17 minutes in total (7 minutes to prepare). There are "listen and repeat", "listen and answer questions" and "answer questions" three parts. Questions are like using 1.5 minutes to 介绍一下你的一个好朋友. The intermediate tests are around 21 minutes (10 minutes to prepare). There are "listen and repeat", "look at a picture and talk about it", and "answer questions". Questions are like using 2 minutes to talk about 你一般是怎么安排你的周末的. The advanced tests are 24 minutes in total (10 minutes to prepare). There are "listen and retell", "read aloud" and "answer questions" three parts. Questions are like using 2.5 minutes to answer a question like 有人认为辩论可以让双方取得一致的意见，有人认为辩论会使双方更对立，你怎么看？
About written tests, I'd suggest you can go to bookstores and take a look at 新汉语水平考试大纲 made by 国家汉办. Each level has its own book including one sample test, so you can see which level is more suitable to you.
One tip that will help you to take HSK tests (or basically any tests created by Chinese people) is to take as many sample tests as you can before the real one, so that you can train yourself to be able to finish the tests in the required time and learn the skills of how to answer questions. 祝你好运！