Hi Starbuk,
Technically, it means that a person will complete 4 full cycles of mania -> depression within 12 months. But rapid cycling can be much quicker, like within hours...
Some kinds of AD's will make certain people with bipolar Rapid Cycle very quickly. Especially SSRI's (Paxil, Celexa, Zoloft, Prozac). For me, actually, ALL AD's make me rapid cycle, but I'm very med-sensitive, and quite the exception to the rule....
Hope that helps,
StarCat