revitalize pruning privet hedge
The hedge is about 10 ft high and needs to come down to about 5 ft. It's August 9, 2020, is it too late to cut that much off? I'm concerned about new growth not having time to harden off before winter and the hedge not having enough leaf surface to produce and store food in the roots for the winter and 2021 spring.
Oakland County Michigan
This would be a bad time to prune your privet as it would, as you thought, produce new growth that would not have a chance to harden off before winter. It would probably be best to do a rejuvenation pruning, taking the plant down to 6-12 inches above ground level. This should be done in late winter before new growth begins. Because the shrubs have mature root systems, they will put out a tremendous amount of growth after that pruning. Subsequent pruning should be done in mid-spring after blooming, to keep the hedge at the height you want to maintain. Here is more information on pruning shrubs and hedges: https://extension.umd.edu/hgic/topics/pruning-shrubs-and-hedges