Change in the light schedule...

I've been growing two plants from bag seeds for 3 months. And they are in the 7th week of flowering. They were in 12/12 light schedule until today. I've had to give them 20 hours of dark and I've just turned on the lights.

Should I give them 4 hours of light for today (that's a one time thing)?

Or should I change the schedule and give them 12 hours of light?
I'm a newbie and I'd appreciate any information you'd give.

Thanks in advance...

well just leave the light on 4 12 hours and turn it off again and stay with that schedule,,,more dark than light when flowering aint such a bad thing,just dont make a habbit of it,,,,,peace

Thank you @mrskitz

I've just taken two photos. The photos look a little awkward because of the hps and flash together.