Your custom URL is typically ready for use in less than 60 minutes. However, this depends on your service provider (Comcast, Time Warner, Verizon, etc.) and how frequently they update their domain name lists. Most providers update this every few minutes. International providers sometimes update daily.
Additionally, computers also cache DNS requests (DNS stands for Domain Name System), and is what translates a domain name (e.g. appycouple.com) into an IP address which is used to actually make the connection to a remote server), so it may take longer than that for the cache on your computer to clear (both Mac, Windows and Linux). If your computer is still not detecting your custom domain, we suggest rebooting your computer (2 hrs after registration).
For advanced DNS cache clearing, check this out.