period is late or even more irregular

Brittany • Tue, Feb 28

So I started my period on the 20th of January, three days I spotted, three days I had a heavy flow, and three more days of spotting. Normally my period comes about every 5 weeks, I'm on day 41 of my cycle and not even a glimpse of getting my period. All pregnancy tests I've taken have said negative, how much longer should I wait before getting a blood test done?