The majority of us are carrying inactive C-diff spores in our gut right now, that have been there a long time. Like MRSA, these spores are everywhere; we come in contact with them daily. Taking antibiotics for an infection upsets the natural flora of our gut bacteria, which activates the spores, and they colonize into an infectious state.
Once the infection is diagnosed, you take more specific antibiotics to work on ridding your gut of the infection, and encourage growth of healthy bacteria. It is not unusual to take antibiotics for several months to get rid of an active infection. Once you have had C-diff, you are susceptible to have it again, and again, until the day you die. Treatment will never kill off all C-diff spores in the intestines; they will continue to just be innocent, inactive spores, until something else upsets the gut flora, and sends them into colonization and infectious mode again.
Technically, most of us have C-diff; the difference is whether it has colonized into a full blown infection, or not. Everyone I know personally who has had C-diff had to be treated twice with Vancomycin to get rid of their active infections. For each, it has been anywhere from 2-5 years, and they have not had another colonized infection.