This all depends on whether or not you are hardwired, or if you are using a battery pack.
For hardwired applications, there is no good answer for how much time you can expect, as it will depend on the capacity, age, and ambient temperature that the battery is being subjected to.
For a typical healthy car battery, at an ambient temperature of 65f, we see 6-8 hours of parking mode on a 2-channel system, draining to 12.0V.
We have seen some manufacturers use batteries small enough to only last 2-4 hours.
Please keep in mind that 12.0V is already 50% of your car's battery, as the discharge vs. Stage of Charge (SoC) is not linear.
For battery pack applications, such as the BlackVue B-124, we see 20-24 hours of parking mode, depending on temperature. This is why we typically always recommend a battery pack if possible. This also ensures that your vehicle's battery does not incur any wear and tear.