how many days late to keep testing?

Hi. I am currently 4 days late and have done two tests of different brands and both were negative. I'm wondering from those of you that have gotten pregnant, how many days late were you when the stick finally changed? I used to have very regular periods but the last cycle was 31 days instead of 28. I'm on cycle day 35 now.
I've been TTC for over a year and everyone around me is pregnant with baby # 2 or 3. It's just so frustrating. Of course my husband just doesn't understand why I'm sad (or sometimes angry) when another friend comes out with an announcement. 
Thank you