How many days would be considered "normal" for implantation spotting occur?

My period was due 3 days ago and It usually comes like clockwork. About 4 days ago I started to spot brownish in color. Not red at all and no cramps either. We have been trying for almost a year now and this is the first time this has happened. I really don't have any other symptoms though. I'm aware this would be considered normal, but I don't know how long it should last.