Seeing the bite marks on Pink's legs, Cobra thought many things. First, he thought maybe this woman was already infected and had chosen to come to them even though she knew she would eventually turn. However, the bite marks were too many and based on what he had seen in the recent days, he had come to make a few deductions of his own.
A light scratch at times wouldn't turn someone into a Zombie, but it wasn't always the case and would actually depend on the victim's ability to resist the turning. However, a deep scratch would turn one into a zombie no matter how strong their will is. The time it would take was dependent on two things.
The person themselves, and how deep the wound itself was. However, a bite was what turned people the quickest, but even then, it would depend on where the bite was, the quicker or slower it would turn a person.