I'm only 17!! Help how long should I wait to take the pregnancy test?

My boyfriend and I have sex all the time there for he can't control his ejaculations so he ejaculates in mean. I was told to wait a week before taking a test so I did after the week was over I took the test it was negative. I don't understand how it could be negative though because my bf has ejaculated in me more then 10 times last time was last night.. If the reason is I'am taking it to early then how long should I wait to take another one again?