I recently signed on for an engineering job and before I chose that job I did other onsite interviews with other companies and none asked me to do a drug test on-site. Friends have been in similar situations and none of the companies any of us applied for had on-site drug testing, especially before accepting the job, or the employer accepting me. So I'm guessing you wont have any on-site drug testing unless the job is really law-oriented. I've never had a job that has drug tested me before they gave me an offer. I think most places will do the drug test/physical after they decide that they want to hire you. If it is a good position all the decision making and paper work will take a while, time enough for you to get clean. It takes me ~2 weeks to get clean without using anything (maybe sooner but I've never checked myself less than 2 weeks), I smoke ~3-5x per week, I'm 24 5'7" and 150lbs (female). I suggest getting a few at home drug tests and taking them to see how long it takes, I bet it doesn't take you much longer than 3 weeks (especially if you dilute), but take the at home test to be sure.