In fact, rhododendrons are rarely pruned. But when a rhododendron grows old, bare and sparse, it certainly helps to prune. In this case, they even accept a strong pruning into the old wood and then rejuvenate themselves again.
The best time for pruning is always immediately after flowering. However, you can also cut off shoots in early spring, but then you have to be satisfied with fewer flowers.
Pruning is always at the expense of flowering for the next year, because the plants first use their strength to develop shoots. Support with a suitable fertilizer (leaf compost, special rhododendron fertilizer, etc.) will promote rejuvenation. If you cut back the rhododendron vigorously, it will probably bear hardly any or very few flowers in the coming year.
For a “normal” pruning, it is sufficient to shorten shoots that are too long stemmed. This will encourage them to sprout dormant eyes (closed buds) and branching.
By the way, after flowering, the withered inflorescences are broken out by hand. This is not only for visual reasons, but also prevents fungal infections and diseases.