Electric fuel pumps are not designed nor meant to be run dry. They cavitate and that causes damage to the pump. So running out of gas intentionally is inviting damage to your fuel pump. That said, I had a '04 Prius and before the ECU update to address the issue it hic-cup'd and got into s situation where the ICE did not start (or the Prius thought the ICE did not start). The faulty ECU firmware gave up trying to start the ICE. I was on full charge of the traction battery and was able to drive about two miles on the traction battery alone at which time I was able to pull into a subdivision and call the dealer. At that point the state of charge (SOC) of the traction battery was about 2 bars (25% approx). So my guess is I could have gotten another mile out of the battery before it was depleted. The would be a total of 3 miles. I have no reason to believe it would be much different on my TCH. I've watch the battery level indicator while driving on electric mode.
I'd therefore estimate the traction battery would take the car 3 to 4 (max) miles on electric power alone alone.