Demijohns or other sealed terrariums requires almost no additional water during the course of their lifetime once the initial balance of moisture has been properly established. A balanced demijohn will have some moisture on the glass (typically in the morning or when it gets cold), but this should disappear over a 24 hour period. If this is the case, your demijohn requires no additional water whatsoever……enjoy!
If your terrarium has large droplets of condensation on the glass, remove the lid and make sure that it receives adequate light and airflow across the opening to aid drying. This process can take days, weeks or longer depending on the initial moisture levels in the demijohn. Once the large condensation has disappeared from the glass, you can replace the cork if you’d prefer (this is not necessary as they can be left open for years). It is normal for droplets to appear on the glass again once the demijohn has been resealed, but keep an eye on it as it should dry out within a week or so. If the condensation persists, remove the cork again and repeat the entire process as outlined above.