I am planning to take up ocjp java 7 certification exam
Do you have any older Java version certification ?. If you have , you can upgrade to ocjp 7. If you don't have any older Java version certification , you need to write two exams OCAJP7, OCPJP 7. Go through our OCAJP FAQ thread for more information OCAJP_FAQ , OCPJP_FAQ , ,OCPJP-Mock-Tests .
To know Exam cleared people experience, Follow below links. Their experiences may help you how to prepare ?
Are 15 days sufficient for preparation?
It totally depends on your understanding level in Java.