Rhododendrons come in an amazing range of sizes and shapes
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.