Heats occur about every 6 months. They last about 3 weeks. A week of swelling and bloody drainage, then a week where she is fertile (she stays enlarged but the drainage turns more clear), then another week of drainage. She CAN be fertile any time during the cycle, so she should be protected from other dogs.
You can protect your floors by having her wear a diaper with a sanitary napkin. They make them in small sizes.
Are you planning on having her spayed?
Watch for signs of pyometra (an often fatal uterine infection) that occurs after a season. She may also experience a false pregnancy which is quite distressing to the dog.
Here's a thread on the issue that will help answer your questions on what to expect ....
What can I expect??