I had a small rock chip that had just started to spider, and then about a week later i got hit by a prety good sized rock making a quarter-sized chip which immediately started to crack in one direction (I could literally watch the crack move down my windshield). By the time I called my local Novus windshield repair guy it had grown to about 10 inches long. He was able to fix both cracks, I can barely find the first one. the second one is slightly more noticable if looked at from different angles but it isn't going anywhere, and my insurance paid for it. I recommend to anyone who has to drive on gravel roads, or where they use gravel on icy roads to have windshield protection on their insurance.
Also, a piece of advice I was given.. if you have a growing crack, don't run your defrost, cracks migrate towards the warmer part of the windshield.
i had this rock hit my windshield last summer on the hwy and nothing happend just this tiny little knick far off to the right. then an hour later i had a crack about a foot long reaching toward my side. i figure ok, let's see how far this'll go... it grew inch by inch for most of fall and one day in december, i decided crank the heat and a couple of minutes later i used the washer fluid and CRACK!! in the blink of an eye, i had a line all the way across the windshield. good thing i was insured as the windshield cost just under a grand to repair.