5 hours maybe the earliest they would open but it doesn't seem to be a guarantee it will open that early
"There are approximately 12,000 parking spaces distributed among 15 numbered lots at AT&T Stadium. In addition, there are nearly 12,000 parking spaces in lettered lots around the Texas Rangers Ballpark that may be used on an event day. Parking areas are generally open five (5) hours prior to Cowboys games. AT&T Stadium is not responsible for and assume no liability arising from accidents, fire, theft, damage to or loss of the vehicle or any article therein while on stadium property."