anti-psychotics don't always negate the effects of stimulants. at times they're co-prescribed to augment each other. of course, dose dependently, they may very well block the effects of the stimulant since it's an anti-dopaminergic and anti-adrenergic drugs. two catecholamine's which stimulants are highly dependent on.
the time it takes for an anti-psychotic to leave your system is dependent on which one(s) you're using. also, a drugs half-life and the time it remains active are not always correlated. diazepam has a 200 hour half-life, yet it stays active for a small fraction of that time period.
with olanzapine, it might block the effects of the dexedrine for a maximum of 24-36 hours in my experience. having said that, you have to ask yourself why you're on anti-psychotics in the first place. if it's to manage the symptoms of schizophrenia, then perhaps leaving them isn't the best idea. if it's for any other indication, then i agree with your sentiment that they're dangerous and destructive drugs and i advise you to have an honest discussion with your doctor on why your on them when they're extremely high risk drugs.
best of luck.