I'm writing from memory here and if I recall correctly it's 27ft or 24ft depending if it's single or dual channel.
On long runways I've waited, out of curiosity, to see them go into RETARD mode and many times this has not happened (-800).
This SPEED OFF method, which after some incidents is now discouraged by Boeing, is supposed to give you underspeed protection (Alpha floor as it appears in the 737 manuals and no, tonight I'm not quoting the exact volume and page). I have tried it on the sim on an ILS, you don't get Vref protection, no selected speed protection either, the A/T Alpha floor will engage the A/T in SPEED mode approaching stall speed. The trim wheel will insist on grabbing your attention for what seems an eternity before the Alpha floor gets any chance to do it's job.
You can read about this feature on the never ending Turkish/AMS thread
Off to sleep