This looks like a case for originate. There is an option on Dial to set an absolute time limit, but it feels more like you want to send a standard message with no through audio after it.
Create a copy of the the first 7 seconds, offline, using something like sox.
If you want to do it the hard way, you will need to use a Local channel, so that you do have a through connected Dial that will timeout.
Note, as currently described, your application is more suited to originate, than to the use of the A option on Dial. Dial is about the through connection, but you seem not to be creating one.