It's best to send out-of-town guests their invites 8 weeks in advance, as this may make allowance for making travel arrangements.
There are more possible choices you may need to consider : the invite comapny may provide this service for an extra fee or send the envelopes to a calligrapher. Organizing a marriage might be awfully stressed, so that the neatest thing to do to help it is to do everything you can ahead. They're going to have worked out and told you what quantity of people they suspect will really attend your marriage and reception. You'll have been confounded at the low number they made.