How SOON after unprotected sex can implantation begin?

Implantation usually begins in about 7 days, sometimes a little longer, depends on the swimming strength of the little sperm.

You see, it takes about 3 days for the sperm to travel up the vagina, through the uterus, up into the fallopian tubes, where the egg is fertilized. Then it takes another 3 - 4 days to travel back down the tubes into the uterus where it implants. Then it can take a couple of days for implantation to actually be complete.