Not very long (two trips); but it is holding up very well.
The very best keel guard method (from what I've read on the Australian kayak forums--those guys do a lot of DIY keel guard protection because of all the reefs) is a two part epoxy for HDPE. Many of them add graphite to the mix for hydrodynamics.
Gorilla tape is cheap and widely available.
From the same kayak forums; it appears Racers tape performs the best