There is a saying that most colds run their course in seven days, but the others take a week. But to get back to the question: I don't know if what seems to be a cold might be a cold, an allergy, or the start of a flu. A number of years ago, I thought I had a cold, but there was some beautiful snow and I had a long x-country ski. It was the flu and I was really sick. Ever since then I rest at the first sign of a bug.