I have a healthy rhododendron, approximately nine feet tall. About how far can I safely cut it back?
What is your reasoning for wanting to prune your mature rhododendron back? Without seeing a photo of it, sometimes it's better to do some limbing from the bottom up, making more of a multi-stemmed small tree from it. Pruning a 9' is possible, but what you often find is it's back at its original height before much time has passed - especially if it is well grown/healthy.
Healthy rhodys will normally respond well to heavy pruning, exceptions are if they are in a considerable amount of shade, or have one of the Falconeri, Thompsonnii series in their parentage, or are one of the smooth barked types - those may not sprout new growth if hard pruned and any severe pruning while possible is not without at least a degree of risk.
If you are going to prune: Rhododendrons have dormant growth buds in the leaf axils (where leaf joins stems). Make your cuts just above a rosette of leaves and new growth will emerge from the dormant eyes there. If you have to cut into a branch below any leaf rosettes, look for faint rings in the bark where there once were leaves - close inspection should reveal small bumps, those are the dormant buds where there once were leaves and cutting just above the rings/bumps should stimulate the dormant buds into growth. If you can't find the rings - make your cuts where you must, then go back when growth emerges and remove any bare stubs that remain above...
Dormant buds on rhododendrons should begin to grow within about a month on smaller stems, 8-10 weeks on taller main stems - give plenty of time if pruning later in Spring for growth to form, mature and harden off before next Fall's weather change again - particularly important in your more severe weather zone.
I have a large mature broad leaf rhododendron. I want to prune it to keep it from getting larger. Some of the buds have bloomed. I understand that I should prune those branches after the blooms fade. But there is already new growth on other branches. If I prune them now will new growth sprout again this season?