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.