If your current score is 600 and your target score is above 800, you can start to practise this book from now. 1 test per week, then increase to 2 tests per week. After 2 months, you can take the real test confidently. Good luck!
Download Jim's 1000 listening comprehension Practice Test Items for the New TOEIC
PDF + Audio: CD1 | CD2 | CD3 | Expression reviews | Possible Answers