Xml-rpc Ip

Hello! I was wondering about something, and couldn't find any previous posts going into enough detail about this. Does anyone know how often the ip address you send XML-RPC calls to from an external server changes? I know that there is a never-changing domain name, but for the purposes of configuring a firewall for outbound communication, I am looking for a reasonable certainly that the ip doesn't change very often. Anyone have any experiences?