Generally an invitation should be mailed six to eight weeks before your wedding. In order to accomplish that you must work from that date back. Determine how long you or your calligrapher will need to address and assemble your invitations. Are there conflicting dates within your six week period like a major holiday that could affect your mailing? For example, you may not want your wedding invitation to get lost in the avalanche of holiday cards.
The time from ordering your invitation to receipt it is very dependent on the actual invitation you select. A simple invitation with flat printing will take less time to produce than an embellished, engraved or letterpress invitation. Allow a couple of weeks from the first to the final proof so that you can carefully check every detail.
Once ordered the invitation can take as little as five business days to as long as 3 weeks to produce depending on the style and the details. Ideally you will order your invitation about 6 months before your wedding date to allow time for the unexpected! If you need an invitation in a hurry it is possible to do that! It may cost a little more, but discuss the options with your stationer.