Once established, callistemon are reasonably drought hardy. It can take up to a year for them to full acclimate in the ground, so I would use the finger test...stick your finger down in the soil and if the soil is dry 2-3 inches down, it is time to water. Soak the plant thoroughly, until water flows freely from the drainage holes...check the plant every 3 or 4 days since it is in a container. remember that full sun and wind tend to dry container plants out rather quickly. After a few checks you will have some idea of how fast the soil dries up and can set up a watering schedule for it. Allow the plant to dry out just a bit between waterings.