When to test again?

So, my periods have been abit out of whack for the past 3 months. My cycle normally (If it works) ranges from 28 to 35 days, but right now iy is at 40 days (Late). I just took a test and it was a negative and just wondering when I should test again if my period doesn't pop up? Or if I should go get blood done. Disclaimer: I'm not trying to get pregnant, I have the paragard IUD.