It's about 5 times longer than previous releases were maintained for, and is an experiment. If there's a need for a longer term support branch, there will be one. It's pointless to start maintaining an 5+ year branch with 0 users and a handful of volunteers, none of whom are paid for doing the maintenance.
So yes, in that context, 15 months is long.
Sadly, that's not code Linus wrote. Nor one he merged. (It's from git, copied from rsync, committed by Junio)