If you over whisk the egg whites and the egg whites are beaten or whisked past the point of stiff peaks, the matrix of proteins will begin to break down and the foam will collapse.
When you over whisk or over beat the egg whites then they will become grainy, watery, and flat and your recipe will not turn out good.
So make sure you whisk or beat the egg whites for only the amount of time you need too.
Most times you only need to whisk egg whites or beat egg whites for around 5 to 12 minutes at most depending on the recipe and what you're actually making with the egg whites that you're whisking or beating.
It usually only takes 10 minutes from my experience to whisk or beat egg whites to the stiff peaks.