Two minutes before and after is correct.
I have cycled for years with a front camera and I can only think of a tiny number of occasions when, after an 'incident' I didn't submit a video because of self incrimination... in other words, I was being as much of a twat as the person attacking me with their car.
I am still searching for a camera. I don't want to spend the hundreds that an Innova system would cost, as 99.5% of the time the super high quality footage will stay on the SD card, so it really doesn't matter how well colour corrected it is, or how high the frames per second are, but I obviously just want footage good enough to clearly record registration numbers, which my cheap as chips, Go Pro knock off ebay special action camera has always done perfectly.