Well, I think it's when the sperm penetrates and fertilizes the egg.
FYI:
Discovery Health "From Sex to Conception"
When an egg enters the fallopian tube from the other end, it produces a scent that drives the sperm cells crazy. They become very excited and begin a process called capacitation, during which they shed certain proteins and become very excited. Both the shedding of proteins and the boost in mobility aid the sperm cell in its quest to penetrate the egg.
When a sperm cell finally makes contact with the egg, the head of the sperm releases enzymes that help it penetrate the egg's exterior. Once a sperm cell does, the two cells -- the sperm and the egg -- form a single-celled unit called a zygote, and the change prevents any other sperm cells from getting through.
The fastest sperm can get to a fallopian tube is about 30 minutes, meaning that the quickest conception could occur following sex is in the half-hour range [source: WebMD]. This means that, following sex, the egg could be fertilized before you've gotten up to get a drink of water.
Conception can occur as many as five days after sex or possibly longer, as strong, healthy sperm can survive for about that many days (and perhaps even longer) in the supportive environment of the fallopian tubes as they wait for an egg to be released, if one isn't already present [source: Harms].
You can click on the next page for lots more information on fertility and conception in far less time than it takes to create a zygote.