It should take about 1 hour at 375°F.  I write this without knowing what size potato, but if you bought them at Costco or got the larger loose russet potatoes many stores carry, that is about how long it will take. If placed on an aluminum tray it cooks faster if the tray is not cold. The potatoes are done when the internal temperature is about 210°F.

Always wash potatoes thoroughly, being careful not to break the skin. Pierce skin with a fork to prevent bursting in the oven. To prevent excessive shrinkage during holding, coat the skin lightly with vegetable oil (though this will prevent the skin from becoming crispy). Follow these guidelines for oven baking:

Use a fork to pierce the skin in the form of an X before serving. Do not cut with a knife as this flattens the surface and prevents the potato from being fluffy. Open the potato just before serving by pressing the ends toward the center and lifting and fluffing the meat of the potato with a fork.