Warning timeframe is very situational ... just this morning I was flying over a rocky coastal location, no where near any buildings, infrastructure or people, CAVOK. Flight was probably no more than 20m AGL at any time and VLOS all the time. I had packed up and about to depart, but taking a few iPhone snaps of the landscape / scenery. A Robinson R44 suddenly appeared over a nearby rocky headland, looked like it was flying close to a normal cruise speed, and within 3-4 seconds flew directly over where I had been flying my MP only 5 min before. I estimate the helo was less than 120m AGL - I've had the MP to 120m (max legal alt in Australia) plenty of times, and have flown fixed wing AC at 500' along a coastline, so I feel I have an educated / experienced estimate. There was no significant noise to muffle the sound of the helicopter (minor wind / wave sound), I was actually looking in the relevant direction at the time. My first thought was "geez, if I'd still been flying and up at 120m, that thing would have gone UNDER my MP before I'd have time to react". The simple fact is that aircraft, approaching at the speed and altitude and heading it was, partially hidden by nearby terrain until last few seconds, was impossible to see with adequate warning time to make avoidance manoeuvres if I'd still been flying.