Succulents and cacti are known to be indoor plants that doesn't need a lot of maintenance.
Depending on the climate you live in, you only need to water it about once every 10 to 14 days (approx. twice a month).
Because it has no real water drainage system, during each watering session, make sure you soak the top 0.5inch of the soil. Do it slowly until you see a little bit of water running out of the 'drainage system' where the rocks are at the bottom of the terrarium.
Try not to down your plants while doing so. Allow the soil to completely dry before watering it again.
You can use different types of items to help water the plants in your terrariums to control the water flow: Turkey baster, squirt bottle, use a water bottle and poke a hole in the lid.
These indoor plants survive in natural bright areas but not in direct sunlight. If the plants are left in direct sunlight they can suffer in the long run and if they don't see any natural lighting at all, they can start to wilt.
Succulents and cacti can last a life time if its well maintained compared to other plants. Depending on the type of plants you have in the terrarium, you should change out the soil once every once or twice year. They normally last 6 to 8 years.
Bright but indirect light. Water but don't over water. Water it when the soil is completely dry.
Simple as that!