Today I checked the distance my prius was able to travel purely on battery.
Drove on PWR mode to charge the battery.
When battery fully charged, converted to ECO mode. Unfortunately EV mode was not activating (maybe due to high battery temperature). But if driven with light foot even ECO mode can be EV.
So, on ECO mode I was able to achieve 3.5km when the battery depleted and engine kicked in.
Since it was night time, some juice was also required by the lights so I guess I could have achieved more distance in daylight.
The battery is 53k km old.