When do I take a test

Heather

I have 2 periods a week. A little over a month ago my birth control failed & I took plan b. I took it once before, 12 years ago, but it didn't work. Tomorrow it'll be 28 days since my last period. When should I start thinking about a pregnancy test? I am very grateful for this forum & the wonderful advice I've received on past posts. Thank you for taking the time to read this!