Its possible to get 6 ounces from 250 watts of light, but it won't be easy.
To do that would require high yield strains, perfect distribution of plants, good pruning, and ideal nutrient mix.
I think 4 ounces would be a more realistic goal for a first time grower with a 250 watt light, and even that will take a bit of work.
In terms of when your lights should be on/off in 12-12, that depends on your personal preference.
Some people keep lights on when daylight normally occurs.
Another way is to try and keep your lights on only when you're home and off at other times. That way you can look inside without interrupting the light cycle, and its less likely that stray light indoors will give you way if you're not home.
If you're asking when should you do the switch to 12-12, that depends on what strain you have, how much space you're trying to fill, and when you want to harvest. Its dependent on your exact grow circumstances, and it make take a bit of trial and error to figure out the best time with your particular strains.
In general, you probably want to plants to show gender before you do the switch, and usually that means they have to be at least 3-4 weeks old first.