How long does it take the new growth on a hydrangea to become the "old" wood that the blooms come off of? One year? Two years?
Just one growing season - the growth that was produced last year provides this year's flowers. This year's growth (as well as that of any prior years that hasn't been removed) will produce next year's flowers.
Very good question, mollydog4. I had wondered too. Thanks for the info, gardengal48.