OP,
I took both during the summer... Orgo I and II. I aced both of them. I also retained it a lot better than other subjects and trust me, I'm NO genius!!!
It's about as unnatural a process as you can hope to spend your summer going through, but it's doable if you need to.
Now, let's say you're smarter than I am (likely). You won't need to study as much then, but I found that I had to average between 2-4 hours per day Monday through Friday (couldn't do more because I was working early mornings and late nights), then when the weekends came, it was a weekly marathon of about 16-20 hours per weekend... so my study time out of class and lab averaged to 36 - 45 hours or so per week. Now, I probably represent an extreme case, because I read very slowly (I have to in order to internalize stuff the way I like to), and for orgo I used to play with model kits to make sure I locked down stereochem concepts.
I'm fully convinced that you can ace orgo with less work than what I put in, but for what it's worth, that's what I did. Also, I spent the vast majority of that time struggling through problems.
Good luck!