It takes approximately 1-72 hours for your 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 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.
If you are unable to access your site few hours after changing the nameservers you can access your site via the following proxy until your ISP update your DNS records.