HPT confusion

Jane
Hi ladies. I have a quick question. How do you know when's a good time to test? My cycle varies from 29-32 days sometimes longer. Sometimes shorter. And I've been on metformin now for a month and so I'm wondering if that would also alter the regularity of my cycle. I've tested at 28 and 30 days got 2 BFN. did I test too soon? Should I just wait it out? Help!