[Source: DHD | Via CB]
One of our motivational posters at work read “It takes seconds to ruin the trust it takes years to build.” Even though I hate the motivational posters since they make me feel like I live in a communist state (“Big Brother is watching you”, “The Open Hand cannot Grasp Discipline”)… it is quite true.
I think the effort is more dependent on whom you might be dealing with.
You can spend a lifetime trying to teach a mentally retarded person calculus, and they might never succeed in learning it.
While you can spend as little as few years teaching, kind of "normal people", to understand calculus.