Conception. How quickly conception occurs is more of a theological or philosophical question than a biologic one. If we define conception as evidence of fertilization than within 24 hrs of the addition of sperm to an egg we can see evidence of fertilization; release of the second polar body and two distinct pronuclei one from the egg and one from the sperm. Implantation occurs after embryo transfer.
In Vitro. Conception occurs almost immediately in a test tube. Dating is easy since the date of conception is known.
About 18-24 hrs. When egg and sperm are combined in the petri dish (not a test tube) with the invitro fertilization technique fertilization can occur over an 18-24 hour period. This cannot be observed to have occurred until the petri dish is removed from the incubator after approximately 18-24 hours from the time they were placed together. The exact time cannot be definitely determined..