How long can I stay in the Air Force??

HELP!
I've heard a couple of different answers to this question and would like to hear your input. I just hit my 8 year mark in the Air Force and will be pinning on Major next month. I was told that as an 0-4 that even if I get passed over for 0-5 in the future that I could still complete my 20 years of service ... but I also have been informed that if I am passed over twice for 0-5 and I haven't reached my 20 year mark that I would have to separate. If I'm doing my math correctly, that would be around my 16 yrs of service mark??