Sean, thanks for the posting.
The LintAlert in theory should work for lengths from 12" to 50 feet, but we have taken calls from one or two installers that indicated the unit would not take a calibration on a duct run that was straight out the back exterior wall. Basically the very very short run, coupled with a very efficient termination hood and likely using the aluminum flex would likely provide such a low back pressure that the LintAlert thinks the dryer is not on. The present firmware in the LintAlert is looking for back pressure above .20, and in a very very efficient run, it is possible that the pressure levels would be so low the unit refuses to capture the value. Future versions of the firmware will likely be set to an even lower number to minimize these false positives.
Your question focuses on run length, when the termination hood style and the type of flex transition hose factor heavily into the calculation as well. The style of termination hood can have a tremendous effect on the back pressure levels. This chart is pretty interesting.
www.dryerbox.com/ratings/efficient.htmHope this helps,