Couple of hours. First I downloaded the clips (or ripped them from YouTube) and then uploaded them to my editing software. There is a multi camera tool which automatically synchs up multiple clips of the same thing based on the audio. Then its just a case of jumping between the clips. I then tidying it all up a bit, added the other clips of Bowie and Iggy and the Pistols and there you have it. Download it into an MP4 and upload it to YouTube. I quite enjoy making these multi camera edits. If anyone can suggest a song or live moment that many people videoed and then uploaded clips to YouTube I'll pull another one together.