Generally, moisturizing twice daily post cleansing (in the morning and at night) is considered a standard and reasonable practice. However, if you have extra dry or flaky skin, you may need to moisturize it frequently as compared to someone with oily or combination skin.
This also holds true during winter or when the humidity drops. Even then, over-moisturizing can have adverse effects on your skin, especially if the moisturizer is not non-comedogenic and oil-free.