one good option is an optocoupler that no only isolates ignition circuit but also is an easy way of stepping down 12V levels to what is suitable for your microcontroller [open-collector or logic output] ..
If the signal looks like a square wave when it passes through the optocoupler it will also look like a square wave ..
You can connect it directly to microcontroller's interrupt pin (see attached picture) and just count transitions over 1sec or 1min ..