I was required to take a pth blood alcohol sample to determine whether i was drinking. how long after drinking does it take to get a negative?

Depends: On how much you were drinking, how long it was since you had your last drink, and to some extent your tolerance to alcohol and your size. If your alcohol level was 80 mg% (0.08 gms/dl), the legal limit, it would take an average drinker between 6-8 hours to metabolize it to zero. If you were at 160 mg%, it would take 14-18 hours, and if you were at 20 mg% (below legal limit), it would be 1-2 hours.