It depends on the kind of test you have to pass.
The typical (5 panel) hair follicle test will only test for commonly abused drugs
and amphetamines. So BDZ would not show on these tests.
But they can definately test for them. Usually, a more extensive (altough more expensive) will also test for BDZ. In fact, if they really want to, they can test for virtually any substance you put in your body in the last few months.
As regards to the detection time, it all depends, again, on the test. They will usually take an amount of hair that will cover a 3 months window. The rate at which you hair grows is really individual-dependant, so sometimes the test will show up drug
use going as far back as 5 months.
Might I ask what is the test for? If it's for an employer, you might want to try and get a prescription for any BDZ, as the test is not quantitative. It would give you a good excuse to test positive, assumong they test for BDZ.