What is the latest you can send out a wedding invitation?
Three weeks before the wedding is ok, although there will probably be some overlap between your requested reply-by date and when they receive the invitation. If the wedding is closer than three weeks, it is best to invite people via phone.
The problem with sending wedding invitations out too late, besides the fact that it may inconvenience your guests by making them rush to make plans, is that they may suspect they are on the B-list. You should be extremely sensitive to doing anything that makes someone assume they were not among your first picks to invite to the wedding. However, sometimes you have to send the invitation out late and you shouldn’t worry too much about it.