Typically, setting up new domains and subdomains, takes less than a couple hours in my experience. I would think that updating the name server entries for an existing domain would take a similar amount of time.
If you're using Windows, your DNS resolver cache is probably still pointing to the old IP address. You can clear the cache by going to the command prompt and entering: ipconfig /flushdns
Then refresh your browser and see if you can now resolve the new address.