Prune in early spring before the plants leaf out. Prune a maximum about 1/3 of the hedge at a time.
Why we don't prune in fall: Plants are beginning to move into dormancy for winter months, and pruning now would prompt the plant to put out new growth where cut. This new growth would not have enough time to become dormant and would likely be killed by winter cold.
Here is a good publication from a Wisconsin nursery about privet care: http://www.hawkslandscape.com/privet-pruning-winter-care-and-fertilizing/