You should be fine with a 32GB card recording at 1080P @ 30FPS. Just make sure you pick up a class 10 card with 45MB/s write speed. That way you can use the same card in a new Hero 3+ or even later camera without any issues if you ever get one.
This. I got about two (not full) days worth of footage on my black. The camera would run out of juice before the card got filled.