How long before changes I make to the name servers associated with my domain become active?"

When you register your domain, your registration is available immediately in the shared registry.  However the information, particularly your DNS servers that will point your domain, are not uploaded immediately to the root servers which control the actual propagation of the information.  Root server uploads occur twice daily, usually beginning at 0500 and 1700  Eastern Time (US).  After the upload is complete, your name will begin working from some locations.  However, it could be another 24-72 hours before the information is propagated world-wide and functional from every location.

The same holds true for modifications you make to your domain name DNS servers.  It is always a good idea to carefully plan ahead when you are changing web hosting services that there will not be any interruption in the workings of your web site and domain e-mail.