It takes approximately 24-72 hours for a domain to propagate internet-wide, this is due to the large number of networks associated with this process. These networks are controlled by many different entities and update independently.
Keep in mind that many ISP (Internet service provider) providers who "should be" updating their DNS records (at least every 24 hours), but a number of them may not. Therefore within the domain propagation period your site will be viewable to some parts of the world within few hours (even within minutes) but for some countries it will take up to 48 hours depending on the quality of their ISP's.