As a general rule if a CD is conducted then the police will act immediately or very soon after however in some circumstances they will allow the drugs through and put you under surveillance and continue to gather evidence to build a case. This would normally only apply to cases where larger shipments are coming through and they want to take down as many people involved in the operation. I know of a case where this has happened and the surveillance operation ran for several months during which time they continued to receive shipments. Ironically when the hammer came down they only had a few shipments identified, many others were never detected despite them being watched very closely.