1 time a week for core muscles ie..chest, bi's, tri's, back, shoulders, legs etc. Muscle groups that you can work out every other day are abs, calves and forearms. These muscle groups tend heal and go away fater than the core muscle groups.
Make sure you work your core muscles pretty hard to keep them sore for a week. You dont want to work out a muscle that is still sore. If you have been going to the gym regurally for 6 months plus then you might be able to get away with doing core muscles twice a week due to that mucle healing faster and is no longer sore. But remember letting the muscle heal is as important as working them out. If you keep tearing the muscle (lifting weights) it never has time to heal and get bigger.