I think just let it flow naturally. It depends what type of game you're making. If you start in a town and want to create something lively with it, you may actually spend a long time in that town before combat begins. Or you start in a town and are going to explore the world, save it, whatever, you may leave the town almost immediately and start combat then. You might start the game IN a battle, or have a introduction scene followed by a battle on a way to the next location. If it's a dungeon crawler I'd expect you'd start combat within the first few minutes of the game. If it's a game like the Harvest Moon series, you may not have any combat or minimal combat reduce to a small aspect of the entire game. There are so many possibilities I recommend thinking of "What naturally flows next with the way I'm starting out this game? Is this the best way to approach this type of game? Will it be fun for the player?" instead of "I must add combat soon, when should I add it in?".