Four to Six Weeks in Advance is Ideal
Four to six weeks before the bridal shower date is an ideal time to send out the invitations. If most guests are out-of-towners, however, give yourself a minimum of two months in advance to mail out the invites. This gives everyone enough time to RSVP and take care of logistical concerns.
At Least 2 Weeks Before
If the bridal shower guest list is really short, perhaps composed of only 10 people who are local and easy to contact, then mailing out invites two weeks in advance is acceptable. Just make sure you've also advised them by phone or email and that everyone has adequate time to prepare for the party.
Etiquette suggests that bridal shower invites should be sent out at least one month before the date of the affair. This is to allow everyone ample time to prepare: the hostess to plan the event, the invitees to RSVP and buy their gifts and most of all, the bride to fit the shower in her already-tight schedule.