I shoot for a sword to balance one hand width or less in front of the guard. As said, it varies by blade type and by the fittings. Knives can be anywhere from just behind the guard to several inches in front. Balancing the handle mass ( and choice of materials) and pommel weight allow for fine tuning this. Often, making a sword/knife a little heavier ( by adding a larger pommel ), makes it "lighter' in the hand. I have made swords that weighed several pounds and spun around like they weighed only a pound.
I have used this example to explain this phenomenon:
Take a strong yard stick and a can of soup. Tape the can of soup to one end. With one hand, pick up the can end, grasping the can. No problem. Swish it around like a sword. Again, fast and easy.
Now, pick up the other end of the yard stick with one hand. It feels like it weighs as much as a bowling ball. Try to move it around. It is nearly impossible to control. These are the two extremes of sword balance. The real world is somewhere between the two. The smith tries to find the happy medium where the sword has speed and lightness, but still has impact and cutting ability. The same applies for knives, but to a much lesser degree on most knives, as maneuverability and impact are not normally the main features.
Stacy