Too many unknowns.
You need to try empirically.
Try it one day for 30 minutes. If the car starts up after 30 minutes, maybe go 45 minutes the next day, and keep adding 15 - 30 minutes each day until your car doesn't start.
Then, get a jump start and a battery charge.
Or just listen to music on your phone with earbuds and don't worry about how long the battery can support your stereo.
Frankly, there are too many unknowns for any of us to give you an answer.
Too many variables: capacity and health of battery, power draw of your stereo plus any other accessories that may be running at the time, how loud you play, etc.
More accurate would be to take a clamp current meter and observe how many amps are being drawn from the battery, then calculate from there.
In general, it's not good to run your stereo for extended periods without engine running. Batteries don't like being deep discharged, unless you have an AGM one.
One of those el cheapo voltmeters that you stick in the cigarette lighter would come in handy. Are you hosting a rave?
I have one of those 12v socket multipliers but it doesn't seem super accurate. While I'm cranking my car it goes to 10v but the battery tests good and I'm pretty sure it's not really dropping to 10v.
And I guess I will buy or borrow an amp meter so I can make an accurate calculation.
While I'm cranking my car it goes to 10v but the battery tests good and I'm pretty sure it's not really dropping to 10v.
It is not unheard of for voltage to sag to 10V during cranking since a battery is being asked to deliver a huge load in order for engine to start.
How old is the battery and have you had it load tested recently?
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.