Whenever. The gameplay behind the boss matters way more than which character it is. I don't really like fighting Bowser every couple levels though. I think the 3D games do it best with throwaway boss characters and only a couple of Bowser fights scattered across the game, if even. The only problem with that is when the throwaway characters don't get thrown away, like Petey Piranha and Queen Bee(I'd say Queen Bee counts as a boss due to her gameplay role regardless of her supporting role as a character). I'd like to see the 2D games start doing the same, and lose the worlds consisting of exactly the same formula of levels, castles, and Bowser/Bowser Jr./Koopaling fights every time.