Light hours?!?

Ok,
Here is my question..
Do I give seedlings 18 hours of light and 4 hours of darkness or is it 6 hours of darkness.
I have looked in books I have and heard it from some people but they say different things. I have also searched the forum but I started reading other stuff.

Thanks.

Its 18hrs light and 6 hrs of darkness or going to 24hrs light. Veg only.

Thanks for the reply.

18/6 20/4 24h on

or just between 18-24hrs

i use 20/4 it's best for me that cycle...