Typically, you will need to water your closed terrarium every 3-4 weeks and open it every two weeks to reduce the moisture levels and to let it ventilate. Depending on what plants you have in your terrarium, some will be thirstier than others. A good way to monitor your terrariums drinking schedule is to check the condensation build-up on the glass. If you don't see any condensation on the glass and the soil is dry to the touch, make sure to top it up with a little water. After you water your thirsty little plants, keep the cork off so the container can ventilate and avoid getting mould.