How Much Spotting Is Normal?

K
I have very bad period pain so I'm on marvelon. Sometimes I take the packs back to back to delay my period when I have something important coming up like exams because it gets really bad to the point where I can't concentrate or study properly. 
At this point I'm on the second pack and I'm spotting, which I know is normal, but it's looking a bit excessive.
How much is normal and at what point should I start to worry?
Can my period just override the pill, which it's never done before?