Cycle days???

Jennifer • 💖🐶We're all on a never ending journey. *dog trainer *aquariumist *full time mom *painter *translator
I get my period at the beginning of every month usually, give or take. How many cycle days is that, 24, 30? How do I calculate this? I'm horrible at math. But regardless, I don't think I understand why it is calculated as so. Thanks in advance for the help!