If you are making the invitation yourself it is always a good idea to allow a couple of extra weeks, just incase your schedule changes, and time becomes a little tight. When confirming numbers it is prudent to order extras.
To determine your RSVP date, check with your venue, normally two weeks before the date works well. Then you can confirm numbers in the week before the big day.
When planning a destination wedding, or if you have guests traveling from overseas to attend a Save this Date card will be beneficial for your guests. These can be sent up to one year before the wedding. This will help to ensure the family and friends you want to share your special day with can make the event.