Approximately 100 years. Then it would produce 2 PWh of power (300x average nuclear power plant). Volume 160x Three Gorges Dam. More here: https://prehrada.hrach.eu/en.html
If it takes 100 years to fill the dam and emptying the dam will generate 2PWh, then the expected average power is 2.3GW. That's closer to 3 nuclear reactors.
You will not empty the dam. You need 100 years to move water level 500m up, then you will use the current flow of the Elbe but raised 500m up. That gives you the power increase.
