I messed up and did about 1.5 grams of cocaine in May of 2016. I just had a hair follicle drug test Dec. 17, 2016. I tend to keep my hair short and the only place they could get it was from my arm. What are my chances of failing this test after 7 months of a one night use?

Fairly good I would say, but there are no guarantees of passing, but I suggest you relax since the deed has been done, and it's been 7 months. Such tests are for the detection of heavy drug use that can go back 90 days.