We give zofran at 10-15 minute intervals.I'm not surprised that has been your experience. Studies (and overwhelming anecdotal evidence) show that repeated doses have very little efficacy.
Because ondansetron has a relatively slow onset (10-30 minutes), I think what people are often seeing when repeat doses appear to work is just the first dose finally taking effect. You see the same thing with morphine when you dose it every 3-5 minutes....people end up getting huge doses of it when the initial dose or two probably would have worked fine, had they just let it take effect before re-dosing.
Ondansetron is a great prophylactic, but not a good rescue drug at all.