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All depends on the type of drug test they do, a mouth swab is easily defeated and will only show up if you smoked in the last 12hr or so, drink lots of water b4 the test and try to have a cigarette also as this can give false readings (they ask if you ate/smoked in the last 30min.

Litmus test where you pee on the stick is also only good for few hours (ive actually been smoking on the way to the centre and still passed the test)
Urine sample is a lot harder thc can still be in your system 5 weeks later
ive passed a DVLA urine sample test after 3 week sober and was/im a very heavy user.

Chances are if they do drug tests then you will prob have 1 before you start work.
I was bus driver for 10 years and was heavy smoker of weed every night (never before work)

Blood tests are more expensive and can show up if you smoked in the last 1 year or so
Hair test can last for years before clean but is very expensive to do