The answer is really dependent on your process. Are you putting your hops directly in the kettle, or using a hop sack, are you using a false bottom, are you whirlpooling? You want to find the best method based on the amount and location of hop matter and cold break you expect in the kettle.
For me I pick up off the side a good inch or more off the bottom. I whirlpool, put whole hops directly in the kettle, and use a false bottom. I leave behind at least a half gallon of wort, but it is mostly hop and break material. I found this easier than trying to filter it out based on my system and process.