Healthy azaleas and rhododendrons will usually respond well to hard pruning.

Safest is to remove 1/3 of the material each year over a three year period, but that's a long transition to what you have over what you eventually want and many will cut back all at once. The correct timing is immediately after bloom, and even then you may lose a year of flowers on the slower rhododendrons. Pruning early in the season after bloom gives the new growth the longest possible time to sprout and mature before facing cold again, lessens risk of winter injury.

Azaleas have dormant buds all up and down the length of their stems and will sprout from just below any cut you make.

Growth buds on rhododendrons form in the leaf axils (where leaf joins or once joined stem). If you are making more minor changes in a rhododendrons shape, cut just above a leaf rosette and new growth will emerge from the dormant eyes there. If you have to cut into a branch below any leaves, look for faint rings on the bark which marked previous growth where there once were leaves. Careful inspection should reveal small bumps which are growth buds just under the bark. Make your cuts just above the rings so dormant buds below them will be stimulated into growth.

If you can't find any rings or dormant buds, cut where you must to shape your plant, then go back later and remove any bare stubs to the point where growth emerges.

Dormant buds on rhododendrons should begin to grow within a month on smaller limbs, up to approx 10 weeks on taller main trunks.