Answer: Ideally, a terrarium should be balanced so that it is self watering and never needs to be watered. The moisture inside will provide ample humidity and recirculates through the contained environment as a closed cycle.
In practice, this can be a challenge. There should be a fine, faint mist of condensation on the glass, no more. You may have to vent the terrarium occasionally if you overwatered it initially, or you may have to add a bit of water if it seems dry. Be sure to keep it out of direct sun or it will overheat like a parked car does.
Since it is a closed system, simply sprinkle a bit of tepid water (preferably distilled) on the soil surface if needed.
Have fun with your terarium!
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