Originally Posted by
gaybucks_chip
It depends.
If you're changing the actual nameserver (for example from dns1.someserver.com to dns1.godaddy.com) then it will take 1 to 2 days, but unless the entries have been removed from the old nameserver (or the old server taken offline), it should be seamless... some people will hit the old server and some the new one during the transition
If you're changing the host records in the name server and the DNS, then, depending on how the DNS is set up, it can take as little as 15 minutes.
For example, I moved one of my (nonadult) servers from another provider to NatNet last night. It was a discussion board, and I turned off the discussion board, made the DNS change in Enom's DNS, had NatNet move the data to the new server, updated the DNS, and in about 20 minutes, everything was online and working on the new server.
But unless the old DNS removed the entry for your site from its records entirely, you should NEVER get a "server not found."
Hope that helps!