Over 50% dead you may trim off. I don't even do that... if they come off with only a gentle tug, THEN take them off because the plant is finished using them.
Luckily cannabis is a plant not plagued by tons of weird viral diseases, and most leaves that LOOK diseased, it's a fertilizer, watering, or soil problem. They don't have to be removed; in fact, the plant can take nutrients out of the leaf to put energy into new growth.