How long will my car battery last with car not running and running subs?

I have a 2001 Ford Taurus SES, and a mechanic at MIDAS told me the battery is 50 cranking amps or 50 something less than what the car should have. It's not that the battery is old and low, its a battery a bit smaller than what should be in the car. It was put in by the previous owner. Anyway, I have a JVC aftermarket radio installed, an ORION 1000W amp, and 2 12" Kenwood Subwoofers in the trunk, would it be safe to ever run the radio without the car running? Or would it put way to much strain on the battery/alternator?

There's no way to know the actual power reserve in the battery and the actual consumption so impossible to tell.

With the car not running it will play at high volume about 30-45 minutes max at which time the battery will be too depleted to adequately power the amplifier and subs or start the car. With the car running and the lights and fan blower on, your alternator will be taxed pretty hard and might not be able to keep the battery charged.