The flowering time depends on the species and variety of the rhododendron and the climate in an area. Rhododendrons generally flower from late winter to early summer. Here in the Pacific Northwest region of North America, April and May are prime time for the blossoms to open.
One nice thing about the bushes with large and showy flowers—besides their beauty—is that different flowers on the bush often bloom at slightly different times, which extends the enjoyment for someone who likes to observe them. Planting different varieties of rhododendrons that bloom at slightly different times of the year is also a way to extend the enjoyment.