Trick question, right? Unless you have modified your Prius to be plug-in capable, you can't charge your Prius.
Or do you mean how far can you go on a single tank of gasoline? Your fuel economy will depend on your driving style, and how long a tank lasts depends on just how "full" you filled it with gasoline and how far on the blinking empty indicator you are willing to drive... Most owners tend to fill up by about 1/4 tank, and report in the order of 300-400 miles per tank, but tanks in the 600-700 mile range are not unheard of for risk takers and environmentally-conscious drivers. (Personally I've done over 600 miles on a tank once on my 2001 Classic Prius, but I usually err for safety and average 340 miles per fillup (averaging 45MPG on mine)...)
The Toyota Prius has been on the roads for over 10 years, and this is still listed in Toyota's FAQs about the Prius:
http://www.toyota.com/prius-hybrid/demos.html"10. Do I need to plug Prius in?
Never. Prius automatically recharges its batteries using regenerative braking, or by running the onboard generator. The driver never needs to worry about recharging the battery."
The hybrid battery is charged either through recapturing kinetic energy when you are slowing down by spinning a generator (called regenerative braking), or by taking excess power from the gasoline engine to spin a generator. Currently, no hybrids (without custom aftermarket modifications) are plug-in capable for charging off the mains (so you cannot calculate a distance "on a single charge" as there isn't such a charge). The hybrid battery is used to power the electric motor(s), which can solely propel the car under light load situations, or act as a turbo to the gasoline engine and provide extra power/acceleration on demand. The vehicle may move on either electric, gasoline, or a combination of both engines. Under low loads or at a stop, the gasoline engine may even be turned off. The gasoline engine is kept at its most efficient RPM with the eCVT transmission, so at its best fuel economy, and the electric motors compensate for any extra power demands.
You may also want to view:
http://www.hybridsynergydrive.com/en/quick_guide.html
There are no commercially-available plug-in hybrids on the market so far. (So you cannot plug them in, other than the same gas station pump that most other regular cars use.) Some hobbiests and aftermarket companies have been altering a few hybrids (Prius, Ford Escape Hybrid/Mercury Mariner Hybrid) to make them plug-in capable. Typically this requires adding additional hybrid batteries, besides the ability to charge off the mains.
For more information, check out
http://www.calcars.org/vehicles.html
For cost reasons, unless you are a fleet owner or other high-mileage driver it probably will not be worth the cost of the PHEV conversion for you. (Conversion pricing is high due to startup costs and low volumes, besides the pricing of the needed additional battery packs.)
To note, converting to a PHEV (plug-in hybrid electric vehicle) does not reduce the vehicle's range. It gives the owner the option to recharge the (newly added larger) hybrid battery pack at night (cheap electricity and off-peak electric load which would otherwise be lost). The vehicle would run for a certain distance (longer than stock) on the stored electric power alone, and when the battery pack is depleted to a certain point the vehicle reverts back to its original hybrid self and runs on a combination of the gasoline engine (which will also recharge the battery) and the electric motor. A PHEV would add a greater all-electric range to the existing hybrid, besides the ability (but not the requirement!) to plug it into an electric source.
The displayed charge on the HYBRID battery (seen on the Energy Monitor of the Toyota Prius) rarely ever reaches "full" or "empty" but prefers to stay at a happy medium. The display only shows you the useable area of charge on the hybrid battery, displayed "empty" is about 40% and displayed "full" is about 80%. Actual charge levels of a NHW20 Prius can be seen at:
http://www.privatenrg.com/#Full_SOC
The two bottom pink bars are about 40-47.5% battery charge.
The most common middle four blue bars are about 45-66.5% battery charge.
The two top green bars are about 64-81% battery charge.
Quoting from a June 22, 2004 Toyota press release:
http://pressroom.toyota.com/Releases...T2004062345528
The Prius battery (and the battery-power management system) has been designed to maximize battery life. In part this is done by keeping the battery at an optimum charge level - never fully draining it and never fully recharging it. As a result, the Prius battery leads a pretty easy life. We have lab data showing the equivalent of 180,000 miles with no deterioration and expect it to last the life of the vehicle. ....
To quote from Toyota:
http://www.toyota.com/html/hybridsyn...l/battery.html
"Q: Do they [the hybrid batteries] ever run out of power?
GS: No. A computer makes sure the batteries never discharge completely. They never fill completely, either. "
So yes, it is normal to not see the battery display on the Energy Screen reach green/full very often. (Even when it does read "full" - like after coasting down a long hill/mountain - the battery still isn't charged to full, to prolong the battery life.) It is also not common to see a low/pink battery icon either - usually a long uphill climb or heavy AC use (since the NHW20's AC is electrically-powered off of the hybrid battery, and is not a conventional belt-driven off the engine AC (like on the Classic Prius)).
There is no reason to be concerned at all to see the battery level at either extreme. The car takes care of itself, and you don't have to do anything special. (Turn off the display, or just stay on the more useful Consumption Monitor.) If the hybrid battery is too low on charge, the Prius will just turn on the gasoline engine to recharge it if necessary. If the hybrid battery is too high on charge, it'll bleed off as much energy as needed by running on electric only or occasionally by spinning the gasoline engine with the electric motor (similar to B mode).
I'll note that the older Classic NHW11 Prius does not have any color changes on its battery icon on the Energy monitor... Normally just sits on 3/4, occasionally on 1/2, rarely to either 1/4 or "full", but again, that is just a display of the useable area of the hybrid battery.