How long do dogs stay in heat?

My dog is about a year and a half maybe 2. And I'm pretty sure she's been in heat before cuz her crotch swells up and she constantly licks it. Well just 3 days ago i noticed spots of blood everywhere. I've heard of dogs doing this but nvr had a dog that did. Its really hot outside so i feel bad leaving her out there. How long will she bleed? and how often does this happen to female dogs?