How long can I use my electronic devices for with a power inverter?

I just bought a power inverter so I can plug electronic devices like my laptop and ipod and I'm wondering how long can I use these devices without the engine being started, until the battery is completely drain. I don't want to be stranded in a parking lot. I have a 2007 Sentra so the battery capacity... show more I just bought a power inverter so I can plug electronic devices like my laptop and ipod and I'm wondering how long can I use these devices without the engine being started, until the battery is completely drain. I don't want to be stranded in a parking lot. I have a 2007 Sentra so the battery capacity isn't that big. The power inverter says it will shut itself off when the battery is close to empty but I would like to know an exact number. Thanks
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