I find the rule of every 8 hours of sewing a bit hard to keep track of, but I've noticed that you can hear a definite sound as the needle gets dull, although that isn't the best rule of thumb.
I will take out the needle and compare it to a new one - if it's noticably dull, I don't use it. Also, I will do a 'test push' of it through my fabric. If it won't go easily, pitch it and use a new needle!
I know a lady who bought a tomato pin cushion for her various machine needles and marked the sections 'metallic', 'quilting', 'leather', and other types of needles she used, that way if she needed to change needles between projects, she could do so and keep track of them. She also sharpened them with the small emery 'berry' before using them again. She said this helped her keep track of the dull and the useful. :)