How long can I use a car battery without charging?

I have just bought a multimedia screen for my 2000 Honda Accord. How long will I be able to watch movies on this without the engine running? I was thinking about installing a Voltmeter in my car. Will this be beneficial?

Just watching movie?

12 hours or more probably. Eventually it's going to run the battery down, but a standard car entertainment system only draws a few amps, from a battery that's probably good for 50 amp / hours?

Voltmeter is worthless. What you need is an ammeter to detect current draw. Read the label on the screen. Find the amps or milliamps. That's how much current it draws. Then find out the reserve capacity of your battery. Divide reserve capacity by current draw and that'll give you the amount of time. I'm guessing no more than a few hours.