You specified formal so I'm assuming you are requiring an R.S.P.V. Eight to twelve weeks is appropriate.This gives you time to get a final count on the number of guests attending. If you will be offering your guests a choice pertaining to the menu, this will give your caterer enough time to make arrangements for the menu. Remember that it may be 3-4 weeks before someone RSVP's

Eight weeks is good for a larger more formal wedding. If it is small and low key six would be fine too. Check out sites like www.theknot.com for really good time lines for this and other things.

Invitations should be sent four to six months before the wedding to allow guests ample time to arrange their schedules. If the wedding is small and informal with only close friends and relatives, invitations could be sent as few as ten days before the ceremony

Usually 8-12 weeks is good but if its a small wedding 8 weeks is probably ok.

The printer will tell you all the information you need for wedding etiquette.