I am tripling a cheesy potato recipe. So that would equal 6 pounds of potatoes. This will all be in one large roasting pan. How long would you cook it

It would depend on your oven-that's not a huge amount of food, but if your oven is small or your stove lightweight it might be enough to increase the cooking time a bit- as Nancy suggests, you should test them before taking them out

Unlike increasing a roast poultry or a cake recipe, potatoes don't necessarily take more time when you increase a recipe. As long as you have pan(s) big enough to spread them out.
Don't know if the recipe is a casserole or roasted potatoes with cheese on top, but either way, probably do the same amount of time. Test when nearly done and if not to taste, keep them in a little longer.
So figure the time in the original recipe and a margin of 10-20% more time if needed.