I have always extended the stock birds' daylight 3 weeks before I pair up, and as long as I feel necessary, to give them around 15-16 hours of daylight.
Some say you should give them 18 hours of daylight per day but I have never found this necessary.
However, if you are going to extend the birds' daylight hours by using artificial means, ie, electric light bulbs, then I believe the type of bulb you use is probably more important than the amount of daylight hours you are giving them.
I use special low wattage DAYLIGHT bulbs. You can get them through eBay. They are quite expensive, about £6 each, but they are definitely worth it and will probably last you a couple of seasons. Bear in mind, not only are you getting proper daylight, but the bulbs I am recommending are eco bulbs and therefore cheaper to run anyway.
As for extending the daylight hours for the young birds once they are separated, I have never found this necessary.
Just part the babies from their parents any time after 26 days old. There are a few posts on here about when to separate young birds, but I always take the babies away when they are picking up for themselves and/or jumping out of the nest boxes.
Just put them in the young bird loft as you would normally. Then you have time to think about whether you are going to put the young birds on the darkness system, and that is another conversation entirely.
I hope this helps. If you need any more assistance, drop me a line.
Phil