I did the trail in the TX -> NM direction in under 5 days. I hiked through the last night. Previous people blogging about the route went NM -> TX. But I flew down from WA solo, so it seemed easier to hitch a ride to Pine Springs and finish at the motel in Whites City. Technically I hiked up to the campground near Guadalupe Peak the night before starting (day 0), but even if I had camped at the trailhead, it would only have added 90 minutes or so in the morning.
Since the Sitting Bull Falls are not really on the direct route between the start and finish, I think a fun definition of the "standard route" for this hike is to start from the Guadalupe Peak cone thingy, swim in Sitting Bull Falls, go down into Carlsbad Caverns, and then finish at the gate in Whites City. This does put a restriction on when you start/finish at Carlsbad because the caverns are only open during the day.
For the quoted time I counted from my (first) GPS tracking point at Guadalupe Peak (8:01am), until I got to the gate at White's City (3:52pm). I think those times are Pacific time. If you count my hike in from the Pine Springs trailhead the night before, it's still under 5 days.
http://blog.drgriffin.com.au/posts/2020-01-22-guadalupe-ridge-trail.html