There's no hard and fast rule. Pick the lens you like that works for the DOF characteristics you want and that's fast enough for the amount of light you have. My favorite 35mm portrait lens is a Zeiss 85/1.4 ZE, because it's fast and seems to be designed with the aesthetic of classic large format portrait lenses in mind. For a really smooth background, there's nothing that says you couldn't use a 200mm or 300mm lens for portraits, as long as you've got enough space to move back.