1. Don't panic!

2. I usually partially remove the cover, to allow more air in, when about 20% of the expected number of seeds have germinated. In other words, if I have 100 seeds and expect 80% germination, I partially uncover the flat when 16 (100 x 80% x 20%) seeds sprout. I gradually remove the cover a little at a time over a few days. I don't think there is necessarily a hard rule about this, and leaving the cover on a day or two, or removing it completely now, may not make a huge amount of difference. The important thing is the moisture level in the soil and a little air movement around the seedlings.

Brandon pretty well nailed it but I'm going to elaborate on what he said with an explaination of why removing the dome can be a good idea-

The purpose of the humidity dome is to keep the seeds and growing media moist until seed germination has occured. Once the seeds have germinated, the excess humidity becomes a liability to the seedlings because it prevents air circulation and promotes botrytis, fungus gnats, damping off, and a whole host of other undesirable things.

As mentioned above, there is no single "correct" answer but I believe more harm is done by leaving the dome in place for too long than will be done by removing it. As long as you keep the soil with the un-sprouted seeds moist until the seedlings emerge there is really no reason you need to keep the dome in place.

Scientific explaination for this: During both epigeal and hypogeal germination, the radicle has already emerged from the seed coat prior to the apical meristem reaching the soil surface. This embryonic root is fully developed and can absorb water to replace what is lost through transpiration.

Plain English explaination for this: By the time you see a green sprout sticking up above the soil, the plant has already set a root that can absorb moisture from the soil to keep from drying out and should not need the humidity from the dome to survive.

As with any rule there can be exceptions but most people lose more seedlings to damping off than to drying out.

Good luck!

Velvet, Good luck with your seedlings. I built a light system out of pvc pipe to hold two 4 foot light fixtures. I used lights from the basement with new bulbs. I win some and lose some but it is exciting for the success that I do have. Have fun experimenting! Hope you make it to the plant swap in Chapel Hill. Rhonda