Can spotting last up to a week and how dark can it be?

If you're having un-explained spotting you should seek medical advice as there are so many reasons for this. To answer your question, spotting is common to for 2 or 3 days about a week before your period is due (implantation spotting - where an embryo is implanting itself in the uterus) just prior to your period and immediately after your period. It also sometimes occurs for the day of ovulation. It's usually a very light pink or brown colour and not very dark, but can vary from person to person. If your spotting appears mucosy, seems to be getting heavier or has clots in it, please seek IMMEDIATE medical advice.