I don't recall ever running out of gas.
In the winter I'll fill up when the tank gets down to 1/2. That helps to keep condensation from forming inside the tank. In warm weather I'll go down to 1/4 of a tank.
I've seen the low fuel warning come on in the Prius twice. The multi function display asks "Do you want to navigate to a fuel station? If you select Yes you get a route to the closest gas station. I find a gas station ASAP when that happens.
You NEVER want to run a Prius out of gas. If you keep driving, that will drain the hybrid battery and that could damage the hybrid system. The hybrid system is not designed to be used without the gas engine, I've read where the computer can throw a code and then the engine will NOT start, even after adding gas to the tank. You need to connect to the OBD2 port and know how to clear the code. I suspect that's how Toyota prevents damage to the hybrid system.
(That said... under normal driving conditions, the Prius computer will turn off the gas engine and it will operate as an EV for short distances).