Dauphinoise potatoes: not fully cooked in cream?
You don't want to serve under-cooked potatoes, they will not be pleasant to eat and might give people indigestion. However, there's no reason you would need to as they'll cook in the oven, in fact that's the idea, if you cook them in the cream until they are done they'll break apart. You don't need that step at all really, when I make Dauphinoise potatoes I don't cook them in the cream, I layer them raw in a baking dish, then heat the cream, garlic and a bit of nutmeg in a pan until bubbling before pouring it over the potatoes and putting the cheese on top. Then I bake at 165 fan until a skewer goes through easily, about an hour, maybe longer. Budget 90 minutes in the oven if it's a deep pan. This gives awesome results every time and is less hassle then par-cooking them in the cream, frankly I see no benefit in that method although there's nothing wrong with it. 

In your case as the potatoes are par-cooked then it will be less than 90 minutes, but it will take some time to get the dish up to temperature so it's very hard to say. Once you see the cream bubbling you'll  know it's cooking, check them in 20 minutes and then every 10 minutes after until they pass the skewer test.