Hair Testing: Length of sober time for shampoo

I used the shampoo once to get the job because I smoked out about a month 1/2 before the drug test. I am going on vacation for a week and i want to smoke in the beggining of the trip. It is not a guaranteed test, but what would be the chances of passing a hair test in a week, if i have the shampoo available? They give you 48 hours to get tested after you are randomly selected, so I am just wondering if the shampoo will work after being sober for a week (in case I get pulled when I come back) or if it takes about a month or so of being sober to cover the "toxins" up.