If by "effect" you mean a superposition of quantum states, then it doesn't matter how long it lasts, you still cannot "see" it. Once you make the observation,

If by "effect" you mean a superposition of quantum states, then it doesn't matter how long it lasts, you still cannot "see" it. Once you make the observation, you will see just one of the states. The distribution of results from repeated observations is the quantum mechanical effect you can see.