How do I delete data from a non-functional iPhone?

If it is broken and not functioning it will never be erased, because it cannot turn on to be erased. You can delete the phone from Find my... and that is it.

You can smash the phone into pieces and take the pieces to a recycling center that handles electronic waste.

That will destroy the phone...and data in it...

How long will it take to completely “Erase”?

The erase will occur when......

The battery has ample charge

The phone is powered on

The phone has a good Internet connection and is functioning normally

However.....If the phone is "non-functional" as you indicate, the phone cannot receive the erase instructions and they will remain "Pending" indefinitely.

If you had a passcode on the phone, no one can get at the data on your phone, even if they take it apart and try to rebuild it using other iPhone parts.

Yup, same thing happened to my old iPad. Battery was totally shot and I couldn't power it on at all. Started a thread here to ask the same question. I called AppleCare just to double-check. The only way you can delete stuff is with the device powered on.

You don't. The only way data can be removed from any device is if it is working.