Depends on your domain Registrar. Although most Registrars warn that it can take 12-48 hours for domains to fully propagate throughout the Internet, often it can be a matter of minutes. If you are transferring an existing domain name propagation can take up to 72 hours depending on your old Registrar.
Domain propagation is the period of time, or delay, involved in sending a domain's address information to all the various name servers in the world. Domain name servers (DNS) intentionally keep track of addresses for domains in their memory for a specific period of time. Unfortunately this 'cached' information also stays in the name servers when it has been changed at the domain's registrar. Name servers refresh themselves from once an hour to once every 24 hours. Experience dictates that 2-3 days is a good estimate for the period of time from when a domain's DNS information is changed at the domain registrar to when everyone in the world can see the change.