When deciding how far to extend your curtain rods beyond your windows, there's no one-size-fits-all answer. You need to approach each window on a case-by-case basis and consider several different factors. Your answer will depend on the window's size, its proximity to a wall or to other windows and the fullness of your curtains.
When installing curtain rods, hang the rod so that it extends at least 3 or 4 inches beyond the window on each side, which allows your curtains to overlap both the window and the wall to prevent light from leaking in around the window's edges. If your windows sit closely together and there is not enough room to hang individual rods side by side, hang one long rod across all the windows on the wall.
When you push your curtains open, they "stack" on each side of the window. "Stacking width" is a term used in the decorating industry to describe the amount of rod space you need on each side of your window to accommodate open curtains. While thin, filmy, unlined curtains will not need much stacking width, heavy, full curtains with generous pleating or gathering will require plenty of stacking room so they do not block your window's view even when they are open. According to the experts at Iron Accents, makers of custom iron curtain rods, a good rule to follow is to use a rod that is one-third wider than your window when allowing for stacking width.
If your windows are narrower than you prefer, you can give them the look of wider windows by using rods that extend well beyond the windows' width. When you open the curtains, stack them so that they are completely off the window but no wall is showing between the window and the stacked curtain, which will give the illusion that your window extends to the far edge of the stacked curtain.
Assess every window in your home individually when determining curtain rod length. For example, if you have a wall of windows, you'll need a rod that extends from one end of the room to the other, and you may have no room at all for stacking width. If you are hanging curtains over a French door or sliding glass door, allow for additional stacking width so you can push the curtains completely out of the doorway to properly access or open and close the door.