How long does spotting usually last? a few days or a week?

Highly variable: By spotting, I presume you mean vaginal bleeding that is less than actual menstrual flow. It ranges from just a drop now and then on one's underwear to light bleeding that goes on for weeks. There are numerous causes as well. Many factors contribute: contraceptive method, ? pregnancy, age, other symptoms, sexual activity, regularity of menses. Precise diagnosis requires medical evaluation:)