The short answer to this question is that you should run as often as you can bear; in the beginning however, it's important to avoid injury. The stabilizing muscles in the knees are particularly weak in the untrained, and prone to injury after very little stress. If you sense pain in your knees, it's a good indication that you aught to rest an extra day or two. A study conducted on untrained athletes shows that the short term gains in the early stages of running at high intensity for periods longer than 45 minutes more frequently than 3 to 5 times per week are outweighed by the incidence of injury to the knees caused by over-training. It may help you to know that this state is not permanent, as the muscles around your knees are developed and strengthened. If you'd really like to continue training during the down time of more high impact exercise, yoga and swimming are a great alternative that will make your heart as happy and healthy as you deserve. Keep at it, and live long.