I know I’m going to sound like a broken record but it really depends on the person and their current fitness level. Some people can get away without cutback weeks at all, others need significant cutbacks. Some need a significant cutback when getting into shape but can do with little to none when in shape. Personally, I go on a 4 week cycle with a minor drop in mileage if any on week 2 and, especially as my mileage climbs, a very significant drop (at times dropping by over 50%) on week 4.
I think the key (as so often is the case with many topics related to running) is to find what works for you. If you’re uncertain, err on the side of being slightly too cautious at first and adjust as needed until you find what works best for you. Remember, the cutback week is a time to not quit training but to give your body a chance to rejuvenate before pushing to higher levels of training.