It depends on the result I want to achieve. I usually use either 30 or 20 or both - in order not to use only 30 because 30 and 40 developers can damage your hair if they are used in large quantities. I mix them with the hair dye -usually ash blonde-. I do NOT use a decolor powder it can be very harmful, so I wouldn't recommend you to do that. I leave it on for around 30 minutes. My natural hair is either light brown or dark blonde.
mine is dark brown. I don't remember how my mom did it since she was the one who chose the products and did it on my hair, but make sure you don't put a lot! She did highlights on mine and it went from dark brown to strawberry blond on some areas