In the summer, your open terrarium requires more watering and in the winter it needs little to none.
On the other hand, an open terrarium creates a much drier environment and is more suited to plants that traditionally live in arider, desert-like conditions. Water can evaporate freely out of the top of these types of terrariums, meaning they require some extra watering from you. Less is certainly better that too much - a light watering when the soil is completely dry will more than suffice. In the summer, your open terrarium requires more watering and in the winter it needs little to none.