I keep a tea kettle with boiling water at hand when I'm making polenta. This way if I need to add liquid, it's already at the right temp.
You'll cook it for about 20min I'd say. The cream cheese melts and things incorporate better and better as it heats, so don't fret if it looks lumpy and unmixed at first.
I usually cook it to about the same consistency as Cream of Wheat, but if you cook it harder (which I also do sometimes) you can mold/slice it too. Some people like polenta sliced then cooked in the oven. I've done this with pumpkin polenta, patting crushed pecans on the outside before putting it into the oven to bake.
When it's less firm I serve it like a pudding with a little bit of nuts or dates sprinkled on top of a blob of Hip Whip