I bet I spent around 50 hours over three weeks. ~2 hours for each chapter in the K&B book including questions and the rest on practice exams, reading online materials, coding questions, and posting in forums.
The best thing I did was plan 2 hours for 40-50 practice questions and make sure I understood everything about the question including coding it if necessary. On a test like this, every wrong answer is testing a very specific erroneous thought process and you know you're good when you can say exactly why each answer is wrong.
I missed questions in the varargs/widening/boxing objectives so make sure to look at the "5 golden rules"
thread around here.