When to test again?

I have a fairly regular 30 day cycle. I'm not sure when I O'd, but I knew there was a possibility I could be pregnant, so I tested on CD 28, two days before AF was due & got a neg result. My period never showed so I tested again on CD 34, again neg result. I'm now 6 days late. So I'm just wondering now when, or if, I should test again?