Maroney, you need to read the drug testing info here at this erowid site. Very good info. Go to Plants & Drugs/Cannabis/Drug Testing. Also, I'll be moving this thread to our drug testing area.
http://www.erowid.org/

From what I've read, detection in hair can last for up to a year. But the thing is that when they take a hair sample, they rarely take more than the last three months' growth of hair, so even if you've smoked within the year, they're only going to look at the newest hair growth in the lab sample. If what I've read is correct, hair testing is fairly sensitive. One of our members, FakeBoobsRule, knows a lot about drug testing. I'm hoping he'll see this thread once I move it to the correct area and can comment further.