Common colds average around 7-10 days, but in kids may fade out over a bit longer time.
They often "peak" around the third, maybe up to the fifth day, when the clear, runny nose
often gets thick & colorful - white, yellow, or green. Then, it should start thinning out again as the cold fades. If the thick, colorful runny nose stuff isn't thinning/clearing by 7-10 days, see your pediatrician.