Between 2.5 and 5% depending on the brand.
Keep in mind that the bleach is very much diluted usually about 1/2 to a full cup in an entire bath tub.
It's the other way around... Bleach is so corrosive and toxic, so the best mixture for everyday use is one part bleach to nine parts water.
the only other way to get your hair completely blond is to professionally dye it (eve n then they might bleach it...)
No, you cannot.