No it takes faster to the hair at the root, you want to apply the colour an inch away from the root. Once you have the colour applied from the inch away from the root to the ends allow it to develop for 20 minutes then apply colour to roots and process for another 10 minutes. What colour are you dying your hair? And have you have coloured your hair before? Because it also depends on the colour and whether your hair is virgin or not. I'm a hairstylist I can help you figure out what you need to do.
Normally always 3-4 shades darker at the fresh healthy roots. So depending what cour snd how much of a change you are making to your hair is 1 factor in time it takes Also how healthy your hsir root-end really is is another factor What brand and colour is another
It will be far more vibrant at the roots generally, especially if you are dying on top of old dye. You should start at the roots though and then work down the hair, sectioning into four usually works well unless you have a lot of hair, then you may need assistance to ensure complete coverage.