Padams2359 - I start my indoor seeds with the lights just a couple of inches above the soil containers. As the seedlings sprout, I start raising the lights - always keeping them just a couple of inches above the uppermost leaves. Otherwise, you'll end up with leggy seedlings - you don't want them "reaching" for the light. As far as length of time, I don't like leaving electrical stuff on when I'm not home, so I normally turn the lights on in the late afternoon/early evening & leave them on overnight, turning them off some time in the morning or whenever I have to leave the premises. I'm guessing the lights are on approximately anywhere from 14-16 hours per day.
I don't know what the weather is like by you, but do keep in mind that out of the seeds you've already planted, the cucumber, pumpkin, & watermelon are going to not only shoot up fast, but are going to need to be planted out fairly quickly - like shortly after sprouting. They don't take particularly well to being transplanted once they're up & start vining. The rest can easily go 8-12 weeks if necessary, assuming you repot them as necessary.