The "420" growers use their lights 24/7 and their plants seem to do well and those people know about growing under lights. There was a long thread on the hot pepper forum about lights on 24/7 for the first couple months and the consensus was that the peppers did better with 24 hours of light. Of course those plants are not chives, mint or oregano. I think I would go for the standard of 14-18 hours of light unless you want to experiment.

You're really not using the right search terms if you can't find info on seed starting on the net. There's tons--good and bad. Temperature does matter--good seed packets have a lot of good info on germination temps. The longer light period may not hurt the plants, but also may not be necessary. Chives grown from seed take a while to get to a good maturity--they're easy. Oregano takes a long time to germinate, and mint seeds can be unreliable depending on what type--most are propagated by cuttings to breed true.

First of your question is not quite clear:
--- to "START" seed , meant to sprout and germinate. And for that, you do not need lights, but warmth.

-- after germination, then the seedlings will need light. How many hours n a day? My answer is try to imitate the mother nature; Maximum daylight up to latitudes of 50, is about 16 hours. So I think 16 hours sounds good to me. 12 hour should also be ok. Now that my seedlings are established and I have a long time till plant out, I just give them 12 hours of light, because I don't want them to grow too fast, too big.