All modern cars will have an amount of quiescent current draw to power
things like the alarm, the clock, the radio presets and so on when the
car is parked up and switched off. I've seen it written somewhere that
around 50mA can be considered 'normal'.

My question is - if the quiescent current draw is 50mA (0.05A), how do
I calculate voltage drop per hour?

For instance, if I park the car up at 10pm and the battery is showing
12.5V, with a 50mA draw overnight what will the voltage be at, say,
9am?