There's no precise answer to your question, any more than there's an answer to the question "How often should I water my plants?".

Germination media differ widely in how wettable and water-retentive they are, and that will affect drying out. If the medium was not moist enough to begin with and the plastic dome allows enough air circulation, you will need to add water. If lots of condensation collects on the dome and drips down, the soil mix may be too damp. The only way to know if water is needed is to check (seeing if the containers are lighter, looking at the color of the medium, and best yet, applying a finger at the edge to see if it feels dry).

Eric thanks for the reply. For some reason i thought bone dry for awhile was good while germinating. I read somewhere that you shouldn't water again after you first intially first water the soil when you first sow them. It said the soil can get very dry and that is ok. It was probably a bunch of BS. I put this all together on saturday and yes the soil was pretty dang dry. I got a bottle mister and put as hot of water out of the tap as i could and them misted the soil hoping the temp will help things out a little bit. Maybe i will water every couple of days now.

As mentioned about there is no sure answer to your question.

However for what its worth I just germinated some echinacea seeds and I misted the top of the medium every day. Plenty of seedlings growing happily now.