Depends. . . Fluphenazine is an antipsychotic that can be given as a normal injection w/half-life of 15hrs or a long-acting injection w/half-life of 6-10 days depending upon the chemical formulation. We typically guesstimate that a chemical takes 5 half-lives to get out of your system, so the effect of an injection could be anywhere from a few days to two months.