For closed terrariums, the answer is, almost never! After the initial water (see here), the water cycle should start. As the plants absorb the water from the soil and respire, they will “breathe” excess water into the air. This water will hit the glass, condensate, then trickle down the glass and be reabsorbed into the soil. Depending on the size, and the location, it can be hard to give an exact answer. But generally I water my closed terrariums once a year – just before the summer. Some though, I haven’t watered for years! Just keep an eye on the soil, and if it’s starting to look a bit dry (lighter in colour) give it a water. If there’s moss in there, and it’s starting to look dry or browning, some water can be added. You can also stick a skewer into the soil to see how moist it is.
For open cactus succulent terrariums, you can judge when they need water fairly easily. As a basic rule, I say 2-3 months give it a good water. But, if you keep an eye on the leaves they are a good indication. Squishy leaves generally mean too much water – let the soil dry out. Wrinkly leaves, or leaves drying out completely means the plants are thirsty. Give it a good drink. Always let the soil dry out completely in-between watering these drought loving plants.