Repotting
There is no easy "rule of thumb" for deciding when a plant should be repotted. Some growers routinely repot every year; others every other year. However, it is really the plant that decides when to repot. If it becomes lethargic, take a good look at the soil mix. It may have "broken down" and lost porosity. Rework the soil or repot. Should the roots be pressing hard against the pot, it is time to repot.
When repotting, move the plant up to the next size pot rather than putting a small plant into an oversized pot. Instead of potting up, another solution is to and prune the roots and the top of the plant (about 1/3 of each) and then return the plant to its original pot. Water moderately until the root system is well established.
Plants that are newly potted up should be watered well, but NOT soaked. Unlike established plants that need thorough watering, newly planted hibiscus need to be encouraged to seek out moisture at the bottom of the pots. Overwatering while a plant is still "filling" in the pot with its root system will encourage root rot and discourage deep root growth. Judicious bottom watering is a good way to avoid this problem.