The first week is normally the most crucial. If tiny water droplets or condensation forms on the glass, it's great. If the soil seems soggy and the sides of the glass get too foggy, keep it uncovered for a couple of hours or days to let the soil dry out a bit. If no droplets form at all, it's time to add a little bit of water. For terrariums that contain cactus or succulents, it's ok to let the soil get a bit dry. When your terrarium receives light from only one side (like maybe a window), you should rotate it every couple of days to ensure that the plants don't all grow towards the light source and become leggy. After about a week of this tender loving care with your plants looking pretty good, you can be confident that the cover can stay on for long periods of time. There’s nothing like using rainwater. You can also use tap water that has been collected and stored for a day or two.