In addition to what spuluka said, take alook at this information.
Recommended Training:
Junos Intermediate Routing (JIR) (from classroom standpoint, 16 hours for training and labs)
Junos Enterprise Switching (JEX) (from classroom standpoint, 16 hours for training and labs)
Additional Resources:
Depending on the doucuments you read, 48-60 hours; though you may only need to read the ones relevant to your course JNO-643)
Junos documentation
Junos documentation for EX Series switches
Real-world hands-on experience
Then you will probably need time to go over the material once or twice again depending on your memory, so throw in say another 32 hours.
So say about 100 hours!
Now I have seen students take the courses and at the end of class on Friday they go take the exam and pass. So that would be 32 hours. As spuluka said, your memery s key.