Just rebuilt my index, just over 7 minutes for 1.34 million posts @ 5000 a time and 0 second delay. Could have cranked it a bit higher if need be. As Slavik said, depends your setup and what your hardware can handle.
Just a little update - the slow process was not the Rebuild Index but the Rebuild Users. That one is really slow (over 10 hours). The Rebuild Index take less than half hour to complete.
