The opinion essay part (question 8) of the TOEIC Writing Test is graded on a 0 ~ 5.0 scale, according to the exact same rubrics used for the independent essay section of the TOEFL iBT.
You can see the scoring guidelines in detail at this link:
http://www.onlinenglish.net/iBT/Writing_rubric_Independent_English.html
Best wishes,
~ Jason