When you're making scrambled eggs there's no harm in actually over beating or mixing the scrambled eggs.
You can beat the scrambled eggs for as long as you desire but usually beating the scrambled eggs for 5 minutes or even a few minutes before cooking them is good enough.
Even after you beat the scrambled eggs you need to stir them in the skillet or pan or the beaten eggs will just turn out non scrambled.
You need to use a spoon or fork to stir and move the eggs around in the pan or skillet so that they actually become scrambled.
Otherwise you'll just wind up with a flat cooked egg.
So make sure you use a spoon, fork etc to move the egg around while it's cooking to make scrambled eggs but the beating of the eggs makes the eggs much better.