I'm far from an expert on IV cocaine use, but I would imagine you would be able to get to sleep after about 3 hours. The half life elimination of IV cocaine is 78 minutes according to this study.
However, a lot of cocaine, especially in the US, is cut with things like amphetamines which last much longer and may prevent you from sleeping for much much longer. Something to be very aware of.
We've got some IV cocaine aficionados here somewhere..
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