Your slingbox is in constant communication with slings servers and updates the external ip address of the slingbox if it changes in real time.
As long as the slingbox is making a net connection and you have a port assigned to it's internal ip address (the default is 5001); then you should be able to connect from anywhere at anytime either by the slingplayer app or via the web browser portal.
What you'll want to do however is assing a "static" internal ip addrwess for the slingbox in the router web login menu as well as assign the same static internal ip address in the setup proceedure for internet viewing. this will ensure connection and eliminate the chance of the slingbox grabbing a different internal adress which would stop it from working since the port forwarding is set to a specifc adress when you origionally set it up.
If port 5001 doesn't work for you, try 443 or 80 as they are typically not blocked by the IT departments of most coporations.