Bleeding while on oral birth control

A few weeks ago I had to start a new pack of birth control (oral) because I had dropped a pill down the sink and when I started my new pack of pills as I was told by my pharmacsist a few days later I started having a brown discharge and now it is like a period. I realize that this is expected because I was on a certain week of hormone and then I had to switch to another when changing boxes but how long should I expect this bleeding to continue for?