Your baby shower invitations should arrive no later than 6 to 8 weeks before the party. This gives your guests more than enough time to decide if they can come based on their schedule, or if they want to change their schedule around so they can make the party.
You should also ask your guests to RSVP with the host about 2 weeks before the baby shower. This will give the host enough time to tweak her planning.
Please feel free to check out the baby shower etiquette and the invitation pages on the site for more information on baby shower invites.