18/6 is the typical light cycle, though with autoflowers you can go as far as 24/0.
Any fluorescent tube or LED light bulb should do the job for seedlings. A 6400 K compact fluorescent bulb (i.e. cool white, not warm white but light color isn't critical. Do make sure that your seedlings are getting lots of light--like a bright sunny day--and try to keep the humidity up and not too hot.