Typically cucumber plants only need to be watered once a day, though it does vary depending on the type of climate you are growing them in. If you are trying to grow cucumber plants in an especially hot or humid environment, it will be important to make sure that you water them twice a day.
You will need to be sure that you do not overwater your plants though, as this can cause the leaves to turn yellow and the plants may not produce any cucumbers at all. If your plants are getting too much water, you might want to switch to a different method of watering them, such as a drip irrigation system.