On average, it's about two years from the time a girl first starts to show any breast development to the time that she actually starts her periods. Since girls in the United States start their periods anywhere from 9 to 15 years of age, breast development starting as early as 7 years of age is normal. In general, a girl starts her period in a pattern similar to her mother; if Mom started early, her daughter is more likely to start early. This isn't an absolute rule, though. Factors such as nutrition (overweight children can start sooner than they otherwise might, and underweight children may start later) and illness can influence the onset of puberty.
Keep in mind that, when you discuss body changes and puberty with young girls, your approach may need to be very different if you're talking to an 8-year-old rather than a 12- year-old. The language that you use will need to be simpler, and any reading materials that you use need to be assessed to see if they are written at an appropriate grade level. Also, an 8-year-old may be uncomfortable discussing or showing her body changes to her peers and it's important to make sure your daughter feels comfortable talking with you about any questions that she may have. A girl starting her period at 9 or 10 will probably need a more in-depth demonstration of pads and how to use them than a teenager would.
A girl starting her periods on the earlier end of the range also may need some guidance about how to handle unwanted attention from boys. This attention may lead to her being teased on the playground to dealing with sexual attention from older boys. Girls who reach puberty early are at higher risk for being victims of sexual abuse, so it's important to really discuss with young girls what is and is not appropriate behavior directed at them, and what they should do when they encounter it.
Lastly, even though your eight-year-old daughter may be showing some of the body changes associated with puberty, it doesn't mean she has to give up her childhood. It's important to keep kids as young as possible for as long as possible, and your daughter should still be encouraged to play dress-up, board games, make-believe, and tag... just as much as all the other eight-year-olds.