The tallest rhododendrons are listed at:

ARS Plant Database

The tallest azaleas are listed at:

ARS Plant Database

They tend to grow taller in shade. The taller ones tend to be the tenderest. Some of the tender species grow into trees.

The 6' tall you mentioned is very likely the height you'll get within 10 years. Most of Rhodies grow much much taller if you let them.

For example, I have a hybrid called Lavender Girl that is about 8' tall x 8' wide within 12 years. It is still growing. I am sure it will grow taller than 10 feet. As a matter of fact, I've planted several kinds of Rhodies - Lavender Girl, Mrs Furnival, Mrs Furnival's Daughter and Gomer Waterer to build an informal hedge. Rhodies are a wonderful way to build a hedge.

I am expecting that all of them will eventually grow taller than 10 feet for sure.

Given a little time some can get real tall. This variety is called 'Cynthia'. I planted it 30 years ago. I took the picture with a friend in it and his owner for scale.
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