It's good to keep the lights on 24 hrs when seedlings are concerned. They need relatively warm and humid temperatures as their roots are not fully formed, so moisture uptake is limited. Once the plants reach the vegetative stage (approx. 2-3 weeks from germination), you can put them into bigger containers with better drainage and switch the lights to a more natural light cycle (18/6). Only switch your lights to 12on/12off if you wish to flower your plants. Some people do this from seedling, but yield will be small.