My beers are at 12 psi at 39 degrees, which is a good carb level for beer after about 2 weeks or so. For soda, I carb it at 30 psi at 39 degrees, though. It depends on how carbed up you want it.
I'm not a fan of burst force carbing and shaking because it can lead to foaming and overcarbing. You could try 15 psi for two weeks, and see how you like it. If you need more carb, it's easy to turn it up but very difficult to lower it!