Established frangipani trees almost always get enough moisture from the ground without hand watering. If they are in well drained soil, the weather is hot and there is no rain for more than a month, they will really appreciate the next downpour or hand water.
Small frangipani plants have smaller root systems so they will be not as efficient in extracting moisture from the soil. If the soil dries out in hot dry periods, the first sign of lack of moisture will be drooping leaves.
Frangipanis in pots and bags more attention. Soil in bags and pots heats up and dries out more quickly than the soil in the ground. As a general root, if the soil is dry a few centimetres down from the top, it’s worth watering the frangipani. If the soil all the way to the bottom is dry, the frangipani is likely to be suffering and as time goes by the frangipani will be weakened and start to shown signs of dehydration.