Although a ruptured or deflated Saline Implant may create a "cosmetic emergency" there is no significant medical reason to rush out and replace it. The timing for replacement can be based on what is most convenient for your schedule. However, over time, if the implant is not replaced the scar tissue capsule around the deflated implant may contract. This may make it slightly more difficult to insert a the new implant leading to a small amount of additional discomfort in the recovery period. The end result will be the same.
Web reference: http://thecosmeticsurgeryinstitute.com