Talking About Puberty: Knowledge Is Key!
A good starting point to help answer all those questions is a visit to your pediatrician, especially if your daughter hasn't had a checkup in the last year or so. Your doctor will examine her and can talk to you both about what kinds of periods your daughter can expect at this age.
In the meantime, here are the basics: When girls first start their periods, they may be infrequent or irregular and have very little bleeding. Eventually most girls' periods become more regular, occurring about once a month and lasting about five days. The flow may be heavy on the first day or two before tapering off. The amount of blood lost during a typical period may range anywhere from a few teaspoons to a few ounces.
Mild cramping can be common, but some girls have cramping that's painful enough to miss school. Menstrual cramps can be a normal part of the menstrual cycle, but are sometimes caused by problems such as endometriosis or fibroids in the uterus. Girls with severe cramps, very heavy bleeding, bleeding in between periods or a stoppage of periods should see their doctors.
Another great resource for kids (and their parents!) is the U.S. Department of Health and Human Services' Girls Health site, which covers everything from puberty to relationships to nutrition and emotional health. There is a great section on menstruation that explains exactly what is happening in girls' bodies—there's even a chart of every day of a cycle with an explanation of what's happening and how a girl may be feeling.