Winding a watch is turning the mainspring around its arbor. As the arbor turns, the coils of the spring are pulled from the outside of the barrel, inward. It wraps around the center arbor until it gets to the end of the spring and then you can't turn it any further. It's like wrapping a string around a stick, you can't wrap more than the whole string.
At the end, it's fully wound.
This is like fulling the gas tank of a car. Full is full, you can't add more. And along those same lines, saying a watch is "over wound", which I hear almost daily, makes no more sense than saying a car won't start because the tank has been filled.
On a full wind the watch will run in the ballpark of 30 hours.