How long does it take for a package to get through customs?

I have several vendors in Mainland China that I work with, and I have found that as a rule of thumb, most packages pass through US Customs in a matter of hours without inspection, and even those that have come through with "US Customs and Border Inspection" seals (meaning they have been inspected) generally spend less than 48 hours in the hands of US Customs. Keep in mind that I generally use EMS shipping, and generally most packages I receive come through Chicago, IL - I have had packages that went through New York take several weeks to clear, and have even had them lost entirely at that facility so if you have the means to specify the route of travel, AVOID having anything go through NY if possible, I'm not sure if that facility is heavily overburdened or just that inefficient, but any troubles I have ever had have all originated from that facility (all the CA facilities have been good too, service-wise).