One of our smaller 4oz bottles will usually cover hair right around shoulder length of average thickness, while our larger 8oz size covers hair closer to mid-back length of average thickness. We're one of the only companies to offer dye in an 8oz size, so this larger bottle is often enough for most types and lengths of hair! If your hair is particularly long (usually approaching the small of the back) or more on the thick side though, you may need to get a second or third bottle to ensure you'll have enough!
Our bottles will keep your color good for up to two years unopened (if stored properly in a cool dry place) and up to a year after opening (if stored properly) though, so if you're unsure it's always nice to have extra for touch-ups!