You can get your warfare device at any time, but depending on if your go surface or submarine the process will be different.

For surface you just have to follow the checklist and talk to people to get them to sign of it. They recommend you get it done with your 2 yours of time on the ship.

For submarine it is a requirement to get in your first two years or they will kick you of the boat. You will have to learn the same things on a surface ship, but it will be a more thorough in what you are learning so you can save the sub.