When I have to adjust my schedule like that, I give them a little extra dark, instead of a little extra light. My thinking is: if you give them too much light, you can set them on the road to re-vegging, which, if they're flowering, you probably don't want to do. A little too much darkness won't ever send them on the path to re-vegging.

I also had a power outtage for nearly 3 days, and they got virtually no light during that time. (I used flashlights, to hopefully make them think it was daylight, and basically just keep them on their established schedule.) Surprisingly, after almost 3 days of high heat (peaked at 99.5 degrees F), no ventilation, and virtually no light, they had sustained very little damage. Cannabis plants are actually more durable and tough than I had thought.