With quality cable for USB,
USB 2.0 5 meters (~16 ft as Rick said)
USB 3.0 3 meters
You can add an active (repeater) extension cable. I use these often when I need to extend point to point USB communications. This might be a solution for you.
The other option is to use a USB hub. e.g. for USB 2.0 :
Computer <-> 16ft cable <-> USB hub <-> 16ft cable <-> dongle
Serial is too generic of a statement. Need to know baud/data rate as well as physical interface (voltage levels and single ended vs differential)