What is the current condition of your hair? Some hair types process color faster than others. Damaged and color-processed hair takes half the processing time of healthy, uncolored hair. This is one good reason you shouldn't skip the strand test.
What is your current hair color? Don't rely on the models on the front of the box to determine the hair color you'll get. Instead, match your current color to the chart on the side of the box to see what your results will be.
What do you want your hair color to be? Some changes aren't suited to box color. The most drastic changes are best left to the pros. The best bet is to stick with hues that are no more than two shades lighter or darker than your current color. Anything more dramatic warrants a trip to the hair salon.
Do you have a sensitive nose? All permanent hair dyes have a strong odor, but some use fragrance to mask it. Semi-permanent formulas generally don't smell as bad because they are ammonia-free.