did you germ the seeds before you planted them, or did you just put them into the soil first??
if you germed them first, you should see something in under a week or so max.. if not, i've heard of up to about two weeks max...
you can leave the dome on for a few days simply to keep the soil moist and to allow the roots to grow some more before removing it, but i usually don't even use one on seedlings, strictly clones..
but saying that, for some odd reason i put one on for my current grow, and i'm thinking of removing it either tonight or tommorow, and the seeds just showed today...