It took me a couple of grows to transition from photos to autos. I run my autos at 19/5 start to finish, but even at 18/6 through flower, it's a lot of light. There is a calculation for how much light over how much time (I understand the concept, but have never learned the specifics), and the bottom line is that I ran my 2x4 tent at 300-350w in flower with photos (obviously at 12 hours), but with autos I only run it at around 250w since they are on for significantly longer. If you're running your light hard, like LEDs at 50w/sf, then 12 hours is probably plenty. As a side note, my outdoor autos only got 7-8 hours of direct sunlight and they got pretty big... so with the right 'quality' of light, they can thrive on fewer hours.
I've never seen my plants droop at the end of the day. What kind of lights are you running, and how many watts? What temperature are you keeping your space at? How about the humidity? Just curious, I'm no expert, just trying to keep learning.