It's impossible to generalize because there are too many variables: specific variety/species (some could easily take 20 years), growing conditions, etc. As a very general rule, however, seed to flowering in 5 years would be a very good result. Some dwarf varieties might be a little quicker and I have seen mucronulatum seedlings bloom in 3 or 4 years.

The late Weldon E. Delp of Harrisville, Pennsylvania is famous for getting azaleas and rhododendrons to bloom very quickly from cuttings and seed. This is quite useful to a hybridizer, since the flowers are needed for evaluation and for any further hybridizing efforts. Click below to read the techniques he used, reprinted from the Journal ARS vol. 39, no. 1, Accelerated Propagation of Rhododendrons.

Here is a link that might be useful: Accelerated Propagation of Rhododendrons - Weldon Delp

The second page of the Delp article is below:

Here is a link that might be useful: Page 2 of Weldon Delp article

Thank you for the replies and the link.