How long should you have to wait until you can get breast implants, after having a baby?

Till done nursing... And until breast size has stabilized, as breasts often "deflate" after pregnancy and nursing.
4-6 months. I would recommend you wait 4-6 months after you stop breast feeding. That way the shape/size of the breasts have stabilized. It makes the results of the surgery much more predictable and enables the surgeon to more closely reach your expectations.
Typically 3 months. We normally say to wait 3 months after having a baby before having any cosmetic surgery procedure. The body needs time to heal and recover from giving birth.
>4mos after nursing. I usually advise my patients to wait approximately 4 months after completing nursing before considering breast augmentation. Even then implants may trigger a brief bout of galactorrhea (milk production).