How long did your implantation bleeding last?

My period rolls around like clock work every 28 days. Well, four days ago I had light pink discharge/spotting only when I wiped. The last three days it's been reddish/red brown. Again, when I wipe. Usually morning, accompanied by some cramping that only lasts about 5 minutes and is very mild. The total amount of 'blood' wouldn't even fill half a panty liner and it's pair with a ton of mucus. But since it's now 4 days, I'm slightly concerned. I know it's not my period (due on the 28the) and tests are still negative (and still early) I'm only 8 dpo. Just confused. Advice?