How much Jewish blood/descent do you need in order to be considered "Jewish"?

at least 25%. if you convert to judaism you are not ethnically a jew. you are just a believer in judaism. i am jewish but i do not even follow the religion. and yes jewish is deffinitely an ethnic group. many of the american ones are not true jews honestly. they are only converts. true jewish people are middle eastern or have some blood from jews in the middle east.