"How long before my newly registered domain is available on the internet?"
"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. It could take as long as 24-72 hours before the new domain 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.