+1I suspect that like all things having to do with good riding technique, the frequency with which you check your mirrors is directly linked to the situation you find yourself in. For example, I use my mirrors a lot while stopped in traffic. Don't want run over from behind, not sure that I'd be able to do anything about it but guess I would like to know I'm about to be creamed. I also have to keep an eye on all those BDC around me in traffic. I don't just have threats from in front and behind, but I regularly have threats from both sides as well. I think that in most cases a check every 3-5 seconds seems a bit obsessive to me, but I think I check mine every 10-15 seconds and sometimes more frequently, depends on if I seen something of concern in a previous check.
It helps if I need to make sudden line change. I do it even when driving a four wheeler.