part time to full time

My question is, that i was hired as a part time employee around the end of 8-2008. i recived 40hrs a week for about 2 or so months then my hrs would drop to 20-40 threw january. between jan and mar my hrs were between 8-20. Around March i was asked if i would began to work nights which would give me 40hrs a week threw out the summer. so as of now ive been actualy working 40 plus hrs sence the end of march. almost 3 months now. now my boss is saying that i could be working nights well into november of this year with 40hrs a week. so the question im asking is that sence im geting full time hrs, is there any law stating that they have to make me full time status. im asking this because of course im geting full time hrs but as a part timer i only get half the bennifets half the sick time and half the vacation. thanks for your help