Does anyone know anything about glass?

wondering if anyone knows how long you can drive with a damaged windshield before you have major problems- such as it shattering. a rock hit the windshield today and the impact spot is a little bigger than a quarter but it has “legs” coming from it already. the glass place was closed when this happened and i leave for work before they open. i have to drive about 30km down the highway and hope that they will have time to look at it tomorrow. if not its 30km home again and then back in sometime when they can look at it. is it best to not drive it and wait for an appointment??