DCDC-USB Intelligent DC-DC Converters. How do you plan on managing the screen power on/off with a single power supply? In my setup I have one converter powering the ODROID and the other powering the (12v) screen. The second converter is ignition triggered.
I currently have a Kinetik HC800 power cell which keeps my ODROID in standby until the power supply detects a low current condition from the battery. At absolute BEST I get 3-4 days of standby. There are other factors other than the voltage of the device and the output of the battery that need to be factored. The power supplies efficiency, the amount of power it needs to perform stepdown, heat, it all adds up.
Again, having the experience of the last year and change, I wouldn't worry TOO much how long the android board will last in standby. At best you'll return to your car and instantly have your audio turn on. At worst, it'll take 10-15 seconds to start from a cold boot and you'll have a system like most other OEM infotainment solutions out today.
I'm looking forward to seeing how your project develops!