How long for antipsychotics to leave the body, namely zuclopenthixol decanoate, it has a half life of 19 days?

Basically the half life of a drug just means that it would take about 19 days for half of the drug to leave the system and then after another 19 days half of what was left would be gone and so on. But it often depends on each person. You need to eat and drink properly to help get rid of the drugs. If you were on a cocktail of them it may take you longer to clear them. The best person to ask would be your doctor or pharmacist.