Nice! Been doing this for several months now using the app Daily Roads Voyager I opted for a windshield magnetic mount for my device. Works awesome. The only drawback is overheating of the device when the sun is shining on it while driving for a while.

 

Dammit why the hell didn't I think of this?!
I bought a dedicated dashcam, but the phone would do the job of course.

 

How long can the app record

 

Infinitely...it will overwrite the oldest file depending on the size of the memory, whether it be internal memory, or an sd card.