I don't know the exact length of time that packages actually sit in customs because I don't usually check the tracking # because I know from experience that if the package passes customs, then I will have it anywhere from 6 to 10 days of purchase, so I don't stressing myself out by checking everyday.
Just received a package Wednesday from the states (hardware only), and it took 8 business days to arrive, but no idea how long it was actually in customs. If something takes longer than 10 business days, then there is prolly an issue ;-)