The biological half-life of cocaine is one hour, which means your liver will remove about half of what's left in your body in one hour. That will work the same way as any drug - if you take enough to get a blood level of 100mg at the peak, you'll have about 50mg an hour later, about 25mg after two hours, and so on. I'd expect the noticeable effect (to you or other people) to be over in three or four hours, give or take a little because everyone is different.
The way half-life works is that there could still be a little bit after 72 hours, but it would be 1 divided by 2-to-the-72nd power of what you originally took, which I'm going to call a gazillionth. I think the only way it would still be doing something is if it had just about killed you to start with (which is possible with a large overdose).
PS - it occurs to me that you might be more interested in the timespan during which cocaine use can be detected, rather than how long the drug effect lasts. Like most drug testing, cocaine tests look for metabolites rather than the original substance, and those can stick around much longer. The half-life of the major metabolite of cocaine (which is the hard-to-spell "benzoylecgonine") is about 6-7 hours, which means there could still be a detectable amount in the bloodstream or urine after 72 hours - that might be what you'd heard.
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The way half-life works is that there could still be a little bit after 72 hours, but it would be 1 divided by 2-to-the-72nd power of what you originally took, which I'm going to call a gazillionth. I think the only way it would still be doing something is if it had just about killed you to start with (which is possible with a large overdose).
PS - it occurs to me that you might be more interested in the timespan during which cocaine use can be detected, rather than how long the drug effect lasts. Like most drug testing, cocaine tests look for metabolites rather than the original substance, and those can stick around much longer. The half-life of the major metabolite of cocaine (which is the hard-to-spell "benzoylecgonine") is about 6-7 hours, which means there could still be a detectable amount in the bloodstream or urine after 72 hours - that might be what you'd heard.