Despegue,
Contrary to popular belief, cannabis is not a legal drug in The Netherlands. There is a non-prosecution policy with regards to it, with limits on the amount in possession of an individual that trigger prosecution.
Individuals found under the influence of cannabis while operating / intending to operate a vehicle or aircraft will be prosecuted, as is the case with alcohol.
The police in The Netherlands are trialling a saliva-test which indicates the presence of the active substance in cannabis in your body. They also have the power to arrest and take a blood sample of any person in the case described above if they suspect he/she is under the influence of cannabis.
The use of any unprescribed psycho-active substance is prohibited for aircrew at any time, both by law and usually by employment contract as well.
Suggesting that the use of cannabis is ok during your time off is rather misguided...