The short answer is you can benefit from as little as 2.5 hours of walking per week at a pace slower than 2 miles per hour (3.2 km/hour). However, most doctors recommend a daily brisk 30 minute walk or even a 15 minute run.
The long answer is several studies have been done to evaluate the amount of exercise needed for cardio-protective effects. Here are their summarized findings.
- The Women's Health Initiative Observational Study concluded that walking 2.5 hours per week was associated with a 30% lower cardiovascular risk than not walking.
- The Harvard Alumni Health Study associated walking more than 5 km per week with risk reduction.
- The Railroad Study equated walking briskly for 15 minutes per day (an increase of 73 kcal expenditure from a sedentary baseline) to a 16% reduction in heart disease mortality, and a daily 30 minute brisk walk (or 150 kcal expenditure) with a 27% reduction in the risk.
References: Consultant, May 2006, 589-593, and the above links.