C. serrulata is an interesting plant, that forms a compact and trailing specimen when well grown. It is extremely drought tolerant. The leaves become very thick and succulent under optimal growing conditions, and gradually lose their plumpness if deprived of water. See the close-up image for a good look at the succulent nature of the leaves. Another photo of the species can be seen here.
I received this plant from Jane Robb as part of her distribution of plant material on the occasion of her taking up a new job in London. I admit to some unintentional research with the plant. On one occasion it went more than three months without water of any sort, while in a cool and somewhat humid environment. With considerable guilt, I began watering, gently at first, then working up to a regular regime. The plant responded with aplomb and soon presented me with a great deal of new vegetative growth, as well as a nice crop of flowers.
Obviously, overwatering this species is to be avoided. It should be planted in a sharply drained soil (add some extra perlite or sharp sand to your mix). Keep the soil gently moist for best results, but don't worry about a missed watering. It seems to tolerate fairly cool temperatures, and may do better with a significant drop between day and night temperatures.