From the giant R. giganteum, which is recorded as growing up to 30m (98ft) and can live for several hundred years, to the dwarf R. keleticum which will only grow a few centimetres high.
Rate
As a general rule the lower the final height, the slower the growth. The large-flowered hybrids grow fastest and can increase by 20 cm (8 in) or more each year. Repens hybrids and Japanese azaleas, however, can only produce about 5 cm (2 in) of new growth each year.
Studying the natural habitat of Rhododendrons and Azaleas will give you some idea of the soil conditions required by rhododendrons in your garden.