For the IRS PPS individual line, the answer is before 8am and after 6pm US Eastern.
The below plot was from enQ call data on April 20th 2016.
Once the hold time is in the 30-40 min range, the IRS solves their hold time issue with “courtesy disconnects”, where they hang up on the caller.
If you don’t have time to wait on hold or be hung up on by the IRS, you can speak with an IRS representative almost immediately using enQ. Sign up for a free trial today!