I've been having some mild breakthrough bleeding its not light although its not heavy neither

I've been having some mild breakthrough bleeding its not light although its not heavy neither. I've been on BC for a month now I started bleeding at the end of the 28 pack of pills the bleeding scared me so I stopped my pills for three days, but I was advised to start a new pack of pills to help with the bleeding. My question is how long is this BTT suppose to last?