The Prius is never plugged in to recharge the battery. Battery charge is maintained by normal driving.
The Prius is a "charge sustaining" hybrid vehice. This means that it manages the battery charge itself and keeps it topped up using power from the gasoline engine and/or power recovered during coasting and braking. In practice, you can ignore the fact that the car has a second power source and drive it like a conventional car.
No longer than four weeks. Leaving any car undriven for a length of time requires caution. The battery will be discharged by electrical systems such as vehicle security and the fuel will become gummy and clog the engine. Hybrid vehicles have small 12 volt batteries and the tiny power drain of the security system will exhaust them in weeks instead of months for a conventional car. This is the limiting factor for a Prius. The 12 volt battery, if healthy and fully charged at the beginning, will be completely discharged in about four weeks. Since it is not a good idea to fully discharge a lead-acid battery, it is inadvisable to leave the car for this length of time on a regular basis. Toyota have recommended that the car be turned on at least every two weeks and be left on for half an hour. The engine does not need to run for this time nor the car be actually driven.
You can prevent the 12 volt battery from discharging by disconnecting it or arranging for it to be charged by a solar array or battery tender. A cheap trickle charger can be used, but these tend to overcharge the battery, which will eventually damage it. Disconnecting the battery will clear the radio presets, clock and trip odometers, but not the values displayed on the consumption screen.
With the 12 volt battery taken care of, you next need to be concerned with the traction battery and the fuel. If you are leaving any car for three months or more, it would be a good idea to have a mechanic prepare it, including draining off the fuel. There is less information available about discharge of the traction battery. People have left a Prius for a month and had it start right up, but again Toyota stick to their recommendation of running it every two weeks. Personally, I would feel safe at a month, but I would make sure the traction battery is well charged before I left the car. This means taking the car for a run of at least 15 minutes and preventing it from using the battery-only mode in which the engine shuts off. Road conditions permitting, you can "pump up" the battery a bit by getting up to high speed with moderate acceleration and then slowing back down by coasting or gentle braking ending at the place you'll leave the car.
All material Copyright © 2003 Graham Davies. No liability accepted.