I count around 40 seconds for a barrage to start being bombarded on the map. This is way too long to be of any use to the team. Too often I will call something in and it won't come in time to be of any use in the team's push.
If you're playing coop, and are doing the defending sequence, it will be bombarded at the very end, if not the end of the counter attack, making it useless. For it to be used reasonably effectively you are going to have to break immersion for yourself and call it in during the cap, for the inevitable counter attack, as if you can see into the future.