Increasing light on duration for Autoflower during mid flowering

Auto flowering plants are not wonder plants that can work 24/7!! they're just normal plants with a different trait! I'd say leave it at 18/6 since you're already giving them 6 more hours of light than the usual photoperiod plants and you don't want to cause radiation stress to your girls! If you want to try *with caution* increase the schedule to 20/4, I've heard of some people who grew in that schedule with conflicting opinions and experiences. Hope this helps! Good luck and happy growing :)