You cannot say exact date because it depends on DAG size... and.. dev team decision.. and.. hardfork vote 
When the next scheduled fork occurs, developers may improve Ethereum block find times again.
This would increase daily mining payouts and simultaneously bring the POW stage too a finish earlier.
Alternatively, at the next scheduled fork, developers may decrease Ethereum block find times.
This would decrease daily mining payouts and simultaneously delay the end of POW stage.
But, it is best to leave this kind of fine tuning too the developers
If they decrease the block time from 14 s to 10 s, that means the miners will get 50% coins in the same time.