ISC Chicago can before and after Customs depending on which way the package is being sent.
If you're sending a package overseas from the USA then ISC Chicago would be before customs because the package has to go through the USA sorting facilities and then reach the overseas sorting facilities where it goes through customs there.
However if you're getting a package from overseas such as from China, India etc then ISC Chicago would be after customs because the package has to clear the overseas Customs facility before it goes into the USA.
When a package reaches ISC Chicago the package stays at ISC Chicago for no longer than 24 hours.
ISC Chicago is a post office sorting facility that sorts and sends packages on their way to the destination.
So just like any other USPS sorting facility your package or mail should leave ISC Chicago within 24 hours once it reaches and arrives at ISC Chicago.
In rare cases there have been some packages that got stuck at ISC Chicago for more than a few days and I've heard of a few packages staying at ISC Chicago longer than a month.
But that is rare and most packages stay at ISC Chicago USPS sorting facility for no more than 24 hours.
When I've tracked packages that have arrived at ISC Chicago the package was scanned and left the ISC Chicago sorting facility within 24 hours.