You need to get amp draw you're pulling with the stereo and other accessories when powered. Say 10A to run accessory position, dome lights, stereo.
You need to know Amp-Hour capacity of battery.
You need to decide what is tolerable for the battery. Using the top 20% of the battery capacity is safe. Draining to the bottom 20% will start the car, but shorten life of the battery.
Example: New battery, 45AH. It will run the 10Amp load for 4.5 hours to completely dead. (45/10=4.5). But, it will only take ~54 minutes to reach 80% charge. That's the lowest the battery will comfortably go, for a battery purist. 3.6 hours is about how far you can go and still start the car. For me, running the radio for a car wash, I'd probably go an hour and a half. I'm not a purist - it's meant to be used, and I'm not doing it every week.
As the battery ages, you have less run time available.
Starter batteries don't like deep discharge, though they can do it. It just shortens their life.