I forget where I picked this tidbit up, but typically:
By 10:30, 25% of the daily volume has been traded
By 11:15, 33% of the daily volume
By 12:15, 50% of the daily volume
By 1:15, 60% of the daily volume
By 2:15, 70% of the daily volume
By 3:15, 90% of the daily volume
Additionally...