Effects - Delayed onset?

[Pseudo-leipä]

SWIM might do cocaine for the first time this Saturday. If he does, he will start with a small line and then work his way up, so as to get to know his natural resistance and the strength of the substance. He knows that the onset is supposed to be no more than a couple of minutes. But how long after insufflation is it considered safe to say that a line has not effected SWIM? Is it possible that there could be a delayed onset? SWIM knows that with other substances there have been cases of people assuming a drug has not effected them, due to delayed onset, and then overdosed. SWIM just wants to avoid this kind of scenario.