The vernacular in the industry is known as 'shaving their legs', which means to clean up the underside of the plants so they don't choke themselves or waste a lot of energy keeping stuff alive that isn't getting much light and will never be productive.
If you are within the first three weeks of bloom I think you'll do very well to clean your plants up. There isn't a hard and fast rule as to percentage, but you're generally safe taking up to a third of the plant mass, especially if you haven't done any cleaning up on them before.