Implantation is when a fertilized egg implants itself into the uterine lining, right? So, it would occur after conception (when the egg becomes fertilized), which can occur sometime after ovulation (when the egg is released from the ovary). Ovulation is projected to occur right smack-dab in the middle of a cycle, around day 14. The exact day a person ovulates, however, can depend on that person's natural hormone schedule. "All in all, the process from the moment ejaculate enters the vagina to implantation, what is medically recognized as a pregnancy, tends to take anywhere between around five days to two weeks (read more about the reproductive cycle
here)." Therefore, you can expect implantation bleeding to occur (if it occurs at all - like you pointed out,
IB only occurs in about 25% of pregnancies) about 10-14 days after you become pregnant. You can read more about this sort of bleeding here:
The Period During Pregnancy Misunderstanding.