With several years of C/C++ experience, but no
java experience, it took me about 2-3 weeks to prepare. I used Mala Gupta's OCA prep book as well as K&B SCJP6 as references.
Disclaimer: Actually, it took several months to prepare because I would only study a couple days per week and only for a couple hours each time. I estimate the total hours spent studying was around 100, so that's where I came up with the 2-3 weeks.