When i had to have a medical for a new job that included unspecified urine and blood tests (and it's not like you can ask what they test for without looking suspicious) i got some urine testing kits.
After 30 days off i was still showing positive (previously i was smoking every day), after 31 and the morning of my medical i got an all clear.
It turns out that the test wasn't even for drugs but as part of the BUPA type benefit that was part of my package.
But i read that dilution won't necessarily work, they'll just flag you as suspicious if you have extremely watery pee!