I'm 16 days late for my period, two negative tests. I've been stressed to the max, so I'm putting it down to that. How long can stress delay my period for?

Missed period. Did you check urine from early morning sample? Make sure that you are not missing pregnancy. some times line can be very fine especially if the urine sample not from early morning. Stress can skip periods or can cause irregular bleeding Can miss periods for 1-2 months.