How much studying you need to do depends a lot on your background. If you have studied horticulture/turf in school, you will see a lot of common themes. You'll probably will not have to study real hard if that is the case.
I took the turf exam last year, and it focuses a lot on pesticide labels, PPE, and environmental responsibility. However, there will be some liability questions, and many questions from the supplemental turf manual. Some of the questions on the exam were fair but there were others that I don't think were even covered in the manuals. So, I wouldn't say that the exam is difficult, but you should really pay attention to the supplemental turf manual and the topics above. Good luck!