Traditionally the dates are as follows...
- Save the Date - 1 year before
- Formal Invitation - 8 weeks before
- RSVP date - 3 weeks prior
My recommendations are more like..
- Save the Date - As tradition, 1 year before
- Formal Invitation - 10 weeks before (12 weeks if it's a very short engagement). This allows people time to plan and request holiday's if necessary
- RSVP date - 6 weeks before. This allows the guest a few weeks to get organised and you to accurately budget the meal and comfortably order favours and place cards for the correct number of people which in this economy is essential. It also allows you plenty of time to chase the inevitable non-responses that we all dread.