It depends on the woman. Some have periods that last only about three days, some have periods that last eight days or more. (Average is about 4-7 days). Also, it is a misconception that females bleed "a lot" - it is actually only a few ounces' worth over the span of a week.
The heaviness of the bleeding also varies depending the day. Usually on the first day it starts out light and gets heavier, then by the third or fourth day it becomes light again until it finally gets really light and stops by the sixth or seventh day (again, this is not consistent for all women, but just in general this is how it goes).
Most women get their period every 25-30 days (average 28 days). The first day of her period is considered day 1 of her cycle. She would have her period day 1 through 6 or 7, then go from day 7 to day 28 without her period. Then after day 28 she would get her period again, hence day 1 of the next cycle.
The period itself is not all blood, it is actually mainly tissue from the uterus
that is not needed. The uterine tissue is used to nurture an implanted egg (fetus) when a woman becomes pregnant. But when she is not pregnant, this lining is not needed and hence it is shed every 28 days, when it is no longer any good for an egg to implant in. Once her period is over, a female's uterus will start producing the lining again. She gets her first period at about age 10-16 somewhere (average 12 or 13) and has them once every 28 days until she hits menopause
in her late 40s/early 50s, and of course she doesn't get them when she is pregnant.
There you have your period basics. You can also check out http://www.womenshealth.gov/faq/menstruation
.cfm for more info.