How often is a scheduled TAF updated?

1. 1Every hour, on the hour
2. 2Twice a day, at 0000Z and 1200Z
3. 3Every three hours, at fixed times
4. 4Four times a day, every six hours

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 13: Aviation Weather Services — Terminal Aerodrome Forecasts, page 319